Sennheiser HD660S2 Review

Sennheiser HD660S2 Review

9 February 2023 0 By Petros Laskis

Sennheiser HD660S2 Review

Theme and variations

Pros: + Real sub-bass extension
+ Very dynamic and impactful bass with excellent technicalities
+ Prominent mid-range
+ Extended yet smooth and easy going treble
+ Crystal clear and transparent
+ Visceral and full bodied texture
+ Solid center image and sharp imaging
+ Excellent resolution and articulation
+ Pitch black background and excellent detail retrieval
+ Low distortion
+ Lightweight and comfortable
+ Classic appearance and good build quality
+ Two cables are included

Cons: – Bass tuning is not so neutral
– Mid-range might sound as too prominent and tiring
– Narrow and intimate soundstage without diffuse elements
– A carrying case is not included
– Build quality, materials and accessories are not up to some of the competition
– Increased price for essentially the same design

The review sample was loaned by the local Sennheiser distributor for writing a review for “hxosplus” printed edition.
There was no obligation for an international review but I have decided to share my impressions with the community.
As always I haven’t received monetary or any other kind of compensation and I don’t use affiliate links.
The price of the Sennheiser HD660S2 is $/€600 and you can buy it from all authorized dealers around the world.


Sennheiser HD660S2

Below you will find some useful information about the design principles and the technological innovations of the HD660S2 as provided in the Sennheiser product brochure.
For the full literature you should visit the Sennheiser website.

“This warm, inviting signature combined with extended sub bass makes listening uniquely
pleasant and rewarding. And with comfort this good, it practically begs you to keep going
for endless sessions. That’s how HD 660S2 combines the best of many worlds to give you
an intimate experience you won’t want to live without.
– Natural, relaxed, high-fidelity sound with extra sub bass: Optimized surround and diaphragm with new voice coil with 300-ohm impedance
– 38mm high-performance transducers built in Ireland: Steel mesh damping follows the
driver’s geometry to control air displacement, while the magnetic yoke includes milled air
vents for optimal ventilation, which minimizes distortion
– Uniquely powerful vented magnet system for minimized distortion
– Ultra-light aluminum voice coil with 300-ohm impedance for excellent impulse response
– Intimate sound with great timbre and deep, accurate sub bass

Frequency response 8 Hz – 41,500Hz
Impedance 300 Ω
Sound Pressure Level (SPL) 104dB(1 V)
Total Harmonic Distortion (THD) < 0.04% (1 kHz, 100dB)
Weight 260g

According to Sennheiser, the main differences between the two headphones can be summarized as:

Lighter voice coil by ~10%
Lowered resonant frequency of surround vs mk1
Retuned transducer
Now 300 ohms; higher magnetic force vs mk1 (150 ohms) for speed and blacker backdrop
Same steel mesh baffle damping, same mesh ear cup outer grille
Same ventilated magnet design with better airflow (but less air leakage vs mk1 for punchier dynamics in bass range)
Smoother top end vs mk1, with increase in clarity that feels more natural.


Appearance and fit

The Sennheiser HD660S2 is identical looking to the HD660S and the only discerning difference is the bronze color of the side logos instead of the silver of the original model.
The headphone is entirely made of high quality plastic except the steel spring of the headband and the aluminum outer grills.
Build quality is excellent for this type of construction but it can’t match that of some other headphones like the Meze 109 PRO which is also more luxuriously looking and built with higher quality parts.
The ear pads are made with the same velour outer material as with the whole HD6xx series and they are comfortable but don’t have enough room to house the whole ear while they get quite warm after a while.
The overall feeling is that of a comfortable headphone which is additionally very lightweight, making it suitable for extended listening sessions minus the heat.
The initial clamping force was medium, it suited my head shape well and never felt too strong but your experience may vary.
You can always carefully bend the headband and adjust the clamping force to your liking.
All headphone parts are user replaceable and can easily be sourced from the Sennheiser retailers.



The headphone comes with a simple carrying pouch, two 1.8m cables with 6.35mm and 4.4mm plugs and a 6.35mm to 3.5mm adapter.


Associated gear and power handling

The Sennheiser HD660S2 features a new 300Ω driver with a sensitivity of 104dB(1V), almost the same as the HD650 (103dB), so it is really not that difficult to drive as long as you have an amplifier capable of providing a good voltage swing.
I have tested the headphone with various sources in order to form a better and more comprehensive opinion about it being difficult or not to drive.
The FiiO K7 and K9 PRO (balanced outputs), Schiit Lyr+ and Vali 2, Violectric V380² (balanced output), Feliks Audio Euforia, SMSL HO200 (balanced output) and Topping DX5 from desktop gear while the FiiO M11 Plus ESS, M17 and iBasso DX320 (AMP12) where used as portable sources (balanced outputs).
All of them provided excellent driver control and plenty of headroom with all kinds of music and no one of them felt as lacking in power.
The headphone scales very well and will highlight higher quality electronics but up to a certain point as it cannot eventually compete with really high end headphones, like the Meze Elite and HiFiMan Susvara, in scaling potential.
Still the Sennheiser HD660S2 is sounding like a more expensive headphone, outperforming other competitive headphones, like the Meze 109 PRO or the HD660S when it comes to source scaling.


As expected, the high impedance driver pairs very well with OTL tube amplifiers, like the Feliks Audio Euforia, and the listener is rewarded with a full and high fidelity sound but you shouldn’t expect any tonal shifts at least when using a source with a linear frequency response.
Another thing of note is that the new driver can handle louder listening levels better than the HD660S because of the very low distortion.


Listening impressions

The HD660S2 was burned about 100 hours before listening evaluation.
A sound comparison between the HD660S2, the HD650 and the HD660S is included in the course of the main review and not in a separate section.
All headphones were used with the stock ear pads, brand new for the HD660S2 and like-new for the other two.
The HD650 is modded with the Custom Cans, copper – weight mod that only affects bass technicalities without altering the frequency response.
I have used both the stock and Lavricables Ultimate pure silver cable.


This is the first HD6xx series headphone with real sub-bass extension and not merely harmonics that pretend to act like one.
It goes deep without being boosted but the frequency response in not truly linear, there is still an audible mild roll-off, nonetheless it can now clearly reproduce low pitched instruments, like the double bass or the pipe organ while it can sound very satisfying and extended with electronic bass but don’t expect bass-head levels of boosting.
The bass as a whole is relatively balanced but not strictly neutral and reference tuned, the HD660S2 is following the route of the HD650 with a mildly emphasized bass and upper-bass.
The main difference is that now with the actual sub-bass extension the headphone achieves a low end tonal balance and equilibrium and it doesn’t give the impression of the detached mid-bass hump like it did in the HD650.
The transition to the mids is even, the bass doesn’t bleed into the region nor sound as overlapping them.
On the other hand, the HD660S bass tuning is more neutral, linear and reference-type but unfortunately it is lacking in real sub-bass extension.
Comparing the two of them while listening to classical music, let’s say the second movement of Shostakovich’s 5th symphony, there are take-aways for both headphones.


What happens is that certain low pitched instruments, like solo bassoon or cello, will sound more “in tune” and realistic with the HD660S when the S2 will add an unnecessarily coloring but you will find the S1 is lacking in intensity and extension with various instruments like low basses or timpani that sound a little muted and not as deep reaching.
Technical performance is really amazing, the bass is tight, layered and controlled with plenty of texture and excellent definition.
The S2 is a touch warm and cozy sounding but not dark or veiled, it is way more cleaner and transparent than the HD650 and considerably faster, you can hear every last note and feel the rhythmic pace of the bass line.
They trade blows with the HD660S in technicalities but the S2 is considerably more visceral and full bodied compared to the leaner and drier S1.
There is also a notable improvement into rendering the macro-dynamics, the new driver is punchy and impactful with excellent rise and fall times, it sounds very contrasted and powerful, not to the Focal Clear Mg levels, but this is undeniably the best performer of the 6xx family and maybe even better than the HD800S.

The mid-range has a character and a flavor of its own that differentiates it from the famous HD650.
The S2 has a balanced, almost linear, lower mid-range that is followed by an emphasized center mid-range while it steers away from the upper mids boosting of the HD650.
A very forward, mid-centric tuning that clearly favors human voices, especially contralto, male alto and soprano and gives an extra projection to certain instruments or their harmonics.
Voices get always highlighted and projected in front of the listener who additionally feels very closely positioned to the performers, a type of tuning that might work very well for a lot of people but others might find it fatiguing and tiring in the long run.
When listening to the famous male alto Philippe Jaroussky with the HD660S2 and the HD650 you will find out that his voice sounds more natural and realistic with the latter than the former and additionally the listener feels like actually sitting and listening to the performance instead of hugging the singer.
The HD650 is more sweet sounding and rounded with some extra lushness and elasticity to the mid range compared to the more aggressive and enthusiastic HD660S2.


The second movement of Rachmaninoff’s second piano concerto opens with a series of slow chords in the strings, then the piano enters playing a simple arpeggiated figure and then the main theme is initially introduced by the flute, before being developed by an extensive clarinet solo.
With the HD650 and the HD660S the woodwind sound was more balanced and naturally integrated with the strings and the piano when with the S2 got mildly emphasized and artificially highlighted.
On a technical level, the mid range of the HD660S2 is very transparent, crystal clear and finely articulated with plenty of harmonic variety, natural overtones and a full bodied texture.
The timbre is very realistic, the music gets emotionally connected with the listener who is rewarded with an engaging, organic and realistic listening experience.


The transition to the treble is more smooth than the 650 and the S2 avoids the upper treble peak that many people found as irritating and harsh sounding.
The treble as a whole is really exquisite, the engineers have achieved a tuning that is full of energy and light, extended and resolving yet smoother and with a more easy going nature than the HD660S, you can listen for hours long without getting fatigued.
This is a treble of the highest quality and fidelity, the S2 sounds airy, crisp, crystal clear and ultimately transparent.
Additionally the texture is weighty and rounded, not as dry and thin as the HD660S while it decays in a more relaxed way helping instruments to sound full bodied and realistic in their reverberation in time and space.
The detail retrieval is phenomenal for the category, the driver offers a pitch black background that helps in extracting all the finest particles of the music and adding great micro-dynamic contrast without sounding analytical or artificial.
Competitively, the HD650 is sounding veiled and hazy while the HD660S offers a touch more air and sharpness in the treble but can become a little hot and aggressive in this region.


The soundstage is on the HD650 level regarding width and spaciousness but now it feels more intimate, it positions the listener extremely close to the action, sitting together with the performers rather in front of them and the end feeling is that the music is emanating from the insides of his head.
So we are talking about a relatively narrow and claustrophobic soundscape with a strong center image but with a very sharp, precise imaging and good depth layering so as not to sound congested or one dimensional.
Still the HD660S fairs better than the rest of the family with the more spacious and airy soundstage that is slightly wider and more open while it places the listener in a more realistic and proportional distance from the performers.
Listening to large symphonic works with choruses is more rewarding with the spaciousness and openness of the HD660S than the HD660S2 where the proximity to the performers doesn’t sound realistic while despite the excellent positioning you have to be really concentrated in order to pinpoint every instrument.


Compared to the Meze 109 PRO $799

The Meze Audio 109 PRO is $200 more expensive than the 660S2 but in exchange you get a more luxurious appearance, higher quality and more premium materials, better build quality and a hard carrying case.
The headphone comes with two cables, 1.5m and 3m length, but they are both single ended, you have to buy an extra one if you want to go balanced.
The 109 PRO is more easy to drive and can be used with lower power output portable gear.

The comparison was made with the FiiO K7 and iBasso DX320 from their balanced outputs and with the Lavricables Ultimate pure silver cable.
Volume level was roughly matched by ear.


The 109 PRO has a real sub-bass extension that doesn’t roll-off at all so it sounds deeper and fuller especially with electronic bass tunes and modern music
Then the bass is even less neutral than the HD660S with a linear but well emphasized response up to the lower mid-range.
This is a tuning with plenty of bass and mid-bass that favors the fun and enjoyment factor rather than actual tonal accuracy.
It is not that you can’t listen to classical music but the instruments sound slightly out of tune and emphasized compared to, the already slightly colored, 660S2 which comparatively is more in-tune sounding.
The 660S2 has slightly better technicalities, it is a bit more tight, layered, better defined and controlled but also more drier and not as full bodied and visceral as the 109 PRO.
Both headphones are equally fast, the 109 PRO has the lead when it comes to dynamics, impact and raw power but it can also sound a little hollow and bloomy.
The bass on the 109 PRO will just slightly bleed into the mids but it can’t be said that it is overlapping the rest of the frequencies or clouding the mid-range.
The 660S offers a more even and linear transition into the mids with better overall clarity.
The center mid-range of the 109 PRO is more conservatively tuned, slightly reserved compared to the emphasized 660S2 so it offers a better tonal balance and it doesn’t sound as pronounced and at-your-face, voices are less prominent and better blended.
Then the two headphones follow opposite directions, the 109 PRO has an upper mid-range and treble emphasis compared to the subdued 660S2 that results in a more lively, forward and sharper tuning.
The 109 PRO is more fresh and youthful sounding with an abundance of energy and plenty of luminosity but is sometimes touching brightness territory.
It is not harsh or fatiguing but it is not as smooth and controlled as the 660S2 while it has the tendency to sound thinner, a little metallic and artificial with a rushed decay over time so percussion instruments don’t sound that naturally shimmering.
The 660S2 is more resolving and refined with a treble texture of higher quality that responds better in upstream gear and offers finer micro detail retrieval.
As a whole the timbre on the 660S2 is more natural and realistic throughout the whole frequency range, this is a more serious sounding headphone when the 109 PRO is more youthful, fun, dancing and partying.
One notable difference between the two is the soundstage which is more extended and open sounding in the 109 PRO with extra spaciousness and air around the instruments while it places the listener at a more proportional distance from the performers than the very intimate sounding 660S2.
The 660S2 has a more centered image than the more diffused 109 PRO.
An interesting comparison is that when listening to solo singing voices, like an opera aria, the voice on the 660S2 is projected close to your face and at the center of it, at the front of your nose, when with the 109 PRO it feels as being moved a couple of meters away and at the upper part of your head, at your eyes height.
Sub 1K region is becoming more and more competitive with some headphones that offer great performance for the buck like the two discussed here.
Don’t ask which one is better, this is something that you must decide after carefully considering your listening habits.

In the end

So this is the new kid on the block and before even publicly released the debate has already started whether it is an upgrade, a downgrade, a side-grade or if it is the real successor to the HD650 or the HD660S etc.
As always the answer cannot be simple and one-dimensional because there are a lot of parameters involved.
Outside the Sennheiser ecosystem, the HD660S2 is an undeniably excellent sounding headphone with supreme technicalities and a balanced, realistic tuning that offers plenty of musicality and emotional involvement and as such it directly compares with similar or slightly more expensive headphones like the HiFiMan Ananda Stealth or the Meze 109 PRO.
There is no right or wrong here and as always, listening habits and preferences will help you to choose the right headphone for you.
Back to the Sennheiser family, the HD660S2 is definitely an upgrade because it offers the missing sub-bass extension and it has a more modern and technically capable driver.
But then it has a slightly different tuning than its older brothers which is not necessarily better or worse, it is just new and different.
So, there are great chances that you are going to like it and crown it as the successor of your current HD6xx headphone but there are also equally great chances that you are going to stick with the more neutral bass response and open soundstage of the HD660S or the clearly distinctive and unique mid-range tuning of the HD650.
The HD660S2 is essentially a new variation on the same theme and while this might not sound very exciting we better remember that in music sometimes the variations are actually of higher artistic quality than the original theme.
And Sennheiser, acting as a skilful and inspired composer, has managed to create a little gem variation that, in my opinion, has managed to surpass the original theme.

Test playlist

Copyright – Petros Laskis 2023.