Sennheiser HD 660S2 Review: A New Reference For Headphone Fans

Sennheiser HD 660S2 Open-Back Headphones

Sennheiser HD 660S2 Open-Back Headphones







What We Dig

  • Great Build Quality
  • Nice Assortment Of Cables
  • Warm, Detailed Sound

What To Think About

  • Narrow Soundstage
  • Tight Clamp (Initially)

Discover if the Sennheiser HD 660S2 headphones meet your needs with this in-depth review! Find out what features make them stand out!


The HD 660S2 is a speedier, more dynamic version of Sennheiser’s venerated HD 600 series headphones. If you’ve been looking for an HD 600 headphone with more weight and liveliness in the presentation, then your ship has come in.

Sennheiser HD 660S2 Review: Intro

If you’re serious about sound quality, then you’ve probably heard of the Sennheiser HD 600 Series headphones. These headphones have been around in one form or another since the 1990s and are still revered amongst audiophiles for their neutral refined sound.

However, as musical tastes and technology change, so do the demands placed on headphones by enthusiasts, and Sennheiser has attempted to address this with several iterations of the original 1997 HD 600 model (still available at a discount).

To that end, in an effort to fulfill the hi-fi community’s latest desires, Sennheiser has developed the new Sennheiser HD 660S2 ($599.95), an upgrade to the well-received HD 660S (currently deep discounted) released in 2017.

The company promises that this new 660 headphone will provide listeners with a “detailed yet emotionally stimulating audio experience”, thanks to improved sub-bass tuning. They believe it will be a great way for music aficionados to rediscover their favorite tunes with “renewed energy and intensity”.

While a new HD 600 headphone is always big news in the audio world, there’s a lot more competition in the sub-$1000 headphone space since the time the original HD 600 headphone ruled the roost as a flagship product.

Relative newcomers like Hifiman and Meze Audio have recently put out some excellent products in this price range. So does the HD 660S2 have what it takes to mix it up in this competitive segment? Read on, and I’ll give you the scoop!

RELATED: Audiophile Headphones Buyers Guide (2023)

Sennheiser HD 660S2 Review: Key Features/Specs

  • New warmer sound, deeper sub-bass reproduction
  • 38mm (diaphragm; 42mm overall) dynamic transducers
  • 300 Ω impedance and new ultra-light voice coil
  • Open-back design for detailed imaging and a spacious stereo field
  • Retuned transducer surround (70 Hz) for the ideal balance of impact and clarity
  • Ultra-fine steel acoustic mesh baffle and ventilated magnet to negate turbulence around the transducer
  • Plush velour ear padding and adjustable headband
  • 8 Hz – 41,500 Hz frequency response
  • Total Harmonic Distortion: <0.04% (1 kHz at 100 dB)
  • 104 dB SPL sensitivity (@1 kHz / 1 Vrms)

What’s In The Box

Sennheiser HD 660S2 Review
  • Includes two cables: a 1.8m single-ended cable with a 6.3mm plug, and a 1.8m balanced with a 4.4mm plug (Standard HD600 series thickness and connections; makes HD 600S2 compatible with the glut of aftermarket cables out there)
  • Includes 6.3mm to 3.5mm stereo plug adapter with strain relief (allows for use with smaller headphone amps)
  • Cinched storage pouch (light fabric provides mostly dust and scratch protection-there is no large display/storage box as provided with the HD600S.

Sennheiser HD 660S2 Review: Build/Design

If you’re familiar with the latest incarnations of the HD 600 series, such as the Drop/Sennheiser HD 6XX or the HD 660S, then you know what you’re getting here with the 660S2. They have a somewhat lightweight, plasticky build, but it’s a good quality hard plastic. That, combined with the heavy-duty metal sliders, adds up to a durable build overall.

They have a similar feel to Sennheiser’s studio headphones, which are designed to be somewhat utilitarian and lightly tossed around without much concern regarding their longevity. That’s about how I perceive the 660S2 in terms of toughness.

Sennheiser HD 660S2 Review

In terms of wearability, supportive memory foam underneath the headband, combined with the velvet-covered memory foam earpads, supplies a decent amount of comfort. As with all the HD 600 series headphones, the relatively tight clamping force may lightly chafe some folks (primarily towards the top of the jaw) after a while, but the ability to stretch the metal headband and loosen the clamp has been thoroughly discussed on the internet.

I have a pretty large head, and the clamp wasn’t unbearable for me, but at the same time, I never quite forgot the headphones were there like with other models. That said, the lightness of the frame makes the clamp easier to get used to after a while. (Ed. Note: After a few days, the clamp loosened dramatically, and comfort improved a lot. YMMV.)

On the outside of each earcup housing is an open grille, similar to the original HD 660S, that allows the free flow of sound from the rear of the enclosure. You can also hear everything around you. This time around, the Sennheiser logo, which adorns the grille, is a gold color as opposed to the silver on the 660S, as is the lettering imprinted on the side and top of the headphone.

Externally, this change in color (along with a change in nomenclature) seems to be the only difference between the old and new 660 headphones. That said, Sennheiser has clearly stated that the bulk of the changes on the 660S2 is on the inside, primarily focused on improved transducer airflow and a more refined voice coil.

Increased ventilation around the driver magnet, combined with a lighter, more responsive voice coil than the OG 660S, is said to provide increased “depth and nuance of low-frequency passages,” hence “adding dramatic flair to any type of music”. Additionally, Sennheiser promises “lower distortion and more accurate sound representation” due to the higher impedance (300 ohms vs. 150 ohms on the older model).

HD 660S2 Exploded View

Sennheiser HD 660S2 Review: Sound

Sennheiser HD 660S2 Review

Once the paragon of neutrality, it seems like Sennheiser has gradually given in to the desire of audiophiles requesting a more lively and dynamic HD 600 model. Over the years, they have tweaked the original HD 600 tuning adding a touch more sub-bass here and a pinch more presence there but never really straying too far from the neutral reference underpinnings that made the 600 series what it was.

With the HD 660S, there was some added lift in the upper midrange, which opened up some of the “veil” people talked about in earlier HD 600 models and provided some speed, but there was still a sharp drop-off on the low end. That left some listeners desiring additional energy in that area.

Conversely, the HD 660S2 seems to be the response to that critique and, in turn, could be the furthest departure from the original HD 600 tonal balance I’ve ever experienced. This model, with its enhanced sub-bass, pushes the HD 600 series gingerly into V-shape territory, which does bring in some excitement to the sound, but to me, also takes them out of the “neutral” category to a certain extent.


In my opinion, as far as speed and dynamics are concerned, the 660S2 is the best out of the HD 600 series of headphones. The new model has a nimbleness that the legacy models lack, and it really gets your head nodding to the rhythm when listening to faster tracks.

Both micro and macro dynamics are very well defined, allowing you to differentiate between the gentleness of soft passages and the force of louder passages to a remarkable extent.

As far as detail and resolution go, some of the top-end air and resolution are blunted by some roll-off in the upper treble, but the detail in the mids and low-end does a good job of fleshing out a lot of what is missing from the treble. Overall, this headphone is quite clean, even though it may sound a touch closed in compared to headphones with more emphasis on the upper treble.

Tonal Balance/Perspective

As far as perspective goes, the HD 660S2 generally maintains the intimate performance of its forbears, albeit with a little more openness throughout the midrange.

You feel like you’re right in front of the stage, which can be a bad or a good thing, depending on your preference. I personally prefer a more laid-back perspective (with a larger scale) where you have a chance to hear the music sort of wash over you instead of it being so in your face.

Sennheiser HD 660S2 Review

Regarding Tonal Balance, as I stated earlier, there is a slight V or U-shaped thing going on here, which is new for the Sennheiser 600 series. It’s interesting to hear an HD 6X0 headphone with such weight counterbalanced by the trademark smooth treble and the presence introduced in the previous HD 660.

The new tuning is harmonically rich and has plenty of natural texture from top to bottom, which I like a lot.

Running through the audio band, I found the Bass to be of good quality, both deep and articulate. But I found there was just a bit too much quantity at times. The rumble of the sub-bass became a touch distracting on bass-heavy songs, at least to me. Then again, it’s not up to Beats by Dre levels which is a good thing.

On the other hand, with the majority of music I listen to (the stuff with more sensible bass levels), I still felt there was a little bit of artificial emphasis on the lowest notes, but on the other hand, that was somewhat satisfying as it enhanced the dynamics of the music.

As far as the treble goes, it’s very smooth, which for the most part, helps the sound stay fatigue-free during long listening periods. That said, I personally would’ve liked to hear just a little more lift in the treble to open things up just a little bit more and add a touch more air. That said, I know some people will like the treble the way it is and consider it to be a more refined, audiophile way of doing things. I can respect that.

Due to the lift on the low end and upper mids, the midrange is a tad bit recessed but still presented with good detail and transparency. The fact that they didn’t lift the mid-bass along with the sub-bass keeps the lower midrange from being muddy. The upper mids are a touch elevated, but to a good extent, as it opens up the sound and provides a good amount of presence.


If I had to describe the HD 660S2’s Soundstage in one word, it would be “intimate”. If you’re familiar with its siblings, then this shouldn’t be surprising since that has been a hallmark of the 600 series since the beginning.

Most of the 660S2’s action is between the listener’s ears with a sharp center focus and some more diffuse imaging off to the left or the right. If you prefer wide-open vistas of sound, and the ability to pick instruments and vocals out of the sky, then the close quarters of the S2’s presentation may not be your cup of tea.

Even so, the 660S2 had good depth and separation in the center of the stage, it just sounded like all the performers were standing very close together instead of more naturally spread out.

General Listening/Comparisons

I did some General Listening/Comparisons using Hifiman’s EF400 DAC/Amp combo in combination with my laptop and the TIDAL Desktop App.

Vs. Hifiman Edition XS ($499): As far as comfort goes, the Edition XS had the HD 660S2 beat, hands down. The weight distribution of the XS’ suspension headband and the lighter clamp was definitely preferred over the Sennheiser’s vice grip.

Listening to Honey Harper’s “Ain’t No Cowboys In Georgia,” I found the Sennheiser HD660S2 to have slightly more rhythmic drive than the Hifiman due to the sub-bass emphasis and punch of the dynamic driver. The backing drum played fast with lots of weight. The guitar and vocals soared freely over the top, allowing the song to rock with the requisite force. I just wish there was more air and space between the elements of the mix.

Listening to the same song on the Edition XS provided some of the air and space I was looking for on the Sennheiser, along with the impression that some instruments, like the Guitar and Piano, were more fleshed out due to the added treble detail.

On the other hand, the Edition XS was missing some of the punch and low-end dynamics of the HD 660S2, which took away some of the speed and rhythmic drive. Even so, while the HD 660S2 definitely had more excitement and weight to it, I personally preferred the spaciousness and added resolution of the Hifiman.

Vs. Meze 109 PRO ($799): Once again, when it comes to comfort, the competition wins out over the Sennheiser with a suspension headband to evenly distribute weight and a lighter clamp. However, in this case, the heavier weight of the Meze headphone and its moderately tight clamp narrow the difference in overall comfort. The Meze also wins out in build quality and the quality of included accessories, which should be expected for the difference in price.

Regarding Sound Quality, the Meze beat out the Sennheiser in overall spaciousness and width of soundstage, plus I found the laid-back perspective to be more natural. The Meze also beat out the 660S2 in terms of overall resolution, but not by much, which surprised me.

That said, when listening to the Honey Harper song, the dynamics of the 109 PRO were a bit softer than the dynamics of the Sennheiser, which made the Meze sound a touch more sluggish and less rhythmic. The Meze also has a bit of peakiness in the upper mids that the Sennheiser doesn’t have.

At the end of the day, even though the 660S2 once again had more rhythmic drive and excitement, I thought the open sound and laid-back presentation of the Meze were more natural to me, and I liked it more overall.

Sennheiser HD 660S2 Review: The Wrap Up

By and large, I found the Sennheiser HD 660S2 Open-Back Headphones to have an immediate sound with lots of clarity, detail, and speed. I wouldn’t call it the most natural-sounding headphone, but it does a lot of things well and is fun to listen to. It has a good “all-rounder” type sound that does justice to a lot of different genres.

Sennheiser promised to provide a headphone that gives you a “detailed yet emotionally stimulating audio experience,” and I believe they have succeeded in that regard. Just keep in mind that neutrality somewhat takes a back seat to that. I have to say, I personally prefer headphones that are a bit more spacious with a wider soundstage. That said, if you like the detail, intimacy, and transparency of the HD 660S but also wish for a little more rhythmic weight on the low end, then you should check out the HD 660S2.

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