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70mm x 40mm (inside ear cup)

Sennheiser has built a justified reputation for making no-nonsense headphones that let the sound speak for itself. I’ve been listening to its products since I was a child (HD 414 with yellow pads!) and have owned several models since, including my reference HD 600. Since the naming scheme suggests that the HD 660S model represents a step up from there, this review provided a great opportunity to find out if that holds true. I spent just over three weeks using the Sennheiser HD 660S for a mix of critical and casual listening, and some editing tasks, to see if it deserves a place in your headphone collection.

Editor’s note: this Sennheiser HD 660S review was updated on July 7, 2022, to address how this compares to the HD 599 from Sennheiser.

The Sennheiser HD 660S comes in a large, foam-lined black box with a hinged lid, and includes two 3-meter-long “Y” cables, plus a short adapter cable. The case is not that useful for carting the headphones around, it’s really just meant for storage. You’ll need to look for a carrying case if you’re planning on taking them anywhere on a regular basis, especially considering the open-back design. Also, there are no folding hinges to further compact the headset, underscoring that this is really intended for home use only.

These over-ear headphones have generous padding inside the headband, and luxuriously soft padding around the 70mm x 40mm ear cups, typical of Sennheiser’s similarly priced offerings. The 260g weight feels quite light on my head—it’s very comfortable for long periods of time and heat build-up isn’t a problem. I find the clamping force appropriate, and the small amount of swivel in the ear cups means they follow the contours of my head perfectly. Notches on the headband make it possible to recall your size setting, but only by counting the clicks made by each adjustment of the relatively stiff metal mechanism.

Overall, the fit and finish of this black-on-black headset are top notch, and the silver Sennheiser branding is minimal and tasteful. One minor detail that irks me slightly is the fact that the cables have black plastic tips on both the left and right ear cup connectors. Earlier incarnations of the HD series had a red connector on the right to make channel identification a breeze. The markings on the headphones are also extremely subtle, molded on the inside of the band, directly above the pads. It turns out the simplest way to figure out which way to put them on when the lights are dimmed is to ensure that the silver “S” logo on each ear cup is oriented towards the back of your head.

As a set of open-back headphones, you’ll only want to listen to the HD 660S in a quiet place. It’s definitely not an office-appropriate headset, because your coworkers will hear everything, and of course, you will hear them. Keep the HD 660S in your living room or den, plugged into an amplifier, audio interface, or whatever you use to listen with.

The build quality is good, but these might not survive being sat on, so keep them somewhere safe when not in use.

The cable is easy to replace, and uses the same 2 pin connector Sennheiser employed for decades, so spares should be easy to come by.

Sennheiser provides three different connection options out of the box: one cable terminates in a 1/4-inch TRS jack, which can be converted to the practically universal 3.5mm TRS jack with the short adaptor cable (which adds around 24cm to the already generous 3m cable length); the other cable ends in a 4.4mm, 5-pin TRRRS (Pentaconn plug), which will only be of use if you have an amplifier with a corresponding “balanced” (aka differential) output port, like the excellent hip-dac by ifi-audio. Really, the 1/4-inch cable plus a standard 3.5mm jack cable would have been of more use to most people. An optional balanced cable (CH 650 S) is sold separately for simplified connection to 4-pin XLR connectors, as found on some “balanced” headphone amps.

No, the Sennheiser HD 660S doesn’t necessarily require an amp, although you may find it beneficial to have some extra headroom available to drive the 150Ω load, with its sensitivity of 104dB/V. Sennheiser does state that the magnets used here are more powerful than those in its predecessors, thus making the HD 660S easier to drive to sufficient levels.

By virtue of their unenclosed design, open-back headphones can’t isolate well, which means the HD 660S hardly impacts outside noise. Unsurprisingly, the passive isolation we measured for the Sennheiser HD 660S is not good. Not only will you hear your environment, but anybody nearby will hear your audio too. That’s no problem in a private space, but if you purchase the HD 660S, you’ll need a different set of headphones for less friendly environments.

Editor’s note: this is one of our first reviews to make use of a hover-enabled glossary, based on a consensus vocabulary. You can read about it here.

Listening to the Vitamin String Quartet’s cover version of Girls Like You immediately showcases the HD 660S’ very respectable representation of space in the music, giving the performance a sense of close proximity, while retaining adequate perceived distance from my head. This really is an area where open-backed headphones can shine. Both the “attack” of the plucked strings and the sustained, bowed elements of the melody sound extremely natural. The violin hovering just to the right of my head (coming in at 0:38) feels beautifully rendered.

Switching gears, Close to Me by The Cure (2006 remaster) brings another example of the spatial abilities of the headphones on a simple stereo mix. Experiencing the “localisability” of Robert Smith’s percussive breathing in the far left and right of the image in this headphone could be quite unnerving for anyone who’s only ever heard this track on a car radio, and you may find yourself looking over your shoulder the first time you hear it. Smith was an AMSR pioneer, as it turns out.

Sennheiser uses newly designed 42mm dynamic drivers (having 38mm diaphragms) to power these over-ears. Testing on our B&K 5128 artificial head reveals that it largely follows our in-house “studio” curve, with a few notable deviations. Typically for an open-back design, sub-bass is lacking, leading to a perceived lack of bass depth. The headset’s frequency response shows a gentle bass roll-off below 100Hz (falling to around 12dB below our 0 line at 20Hz), an extremely natural, even midrange, a little under-emphasis relative to our preferred curve from 2-4kHz, and a roll-off in the highs that begins prematurely, just below 10kHz. What all this adds up to is a pretty “clean” sounding headphone with good clarity, but that doesn’t quite have the revealing upper midrange of the HD 600 due to its slightly “darker” tone.

This provides a natural, pleasant listening experience: you might notice new details in your music, but at the same time, it doesn’t fatigue ears prematurely thanks to the slight under-emphasis in the upper midrange compared to many audiophile-targeted headphones. Anyone who listens to a lot of orchestral strings or vocal-orientated music should enjoy the HD 660S. In other words, look elsewhere if you crave thumping bass.

The HD 660S is not a good choice for anyone looking for commuter cans. The open backs of the Sennheiser HD 660S limits where you can listen without igniting the ire of your coworkers or fellow commuters. These belong at home or in a listening room, and definitely not outside. However, if you’re looking at the HD 660S in the first place, you’ve likely already considered all of this.

If you already have open-backed, over-ear “reference” headphones, the Sennheiser HD 660S might be a tough sell. While it doesn’t have the analytical capabilities of the venerable HD 600, and could be described as a little more “laid back” due to the slightly different tuning, the difference could easily be achieved with some basic EQ, if that’s your bag.

The build quality appears extremely similar between the Sennheiser HD 660S and the older HD 600, as is the general design language and comfort level. The main difference is in the frequency response, which is quite noticeable when switching back and forth between the two headphones.

The HD 600 appears to have a slightly clearer sound, thanks to the comparative extra energy in the upper midrange. Conversely, the HD 660S could be said to have a slightly “darker” tone, which may be better for longer, less analytical listening sessions.

Pick up the Beyerdynamic DT 900 PRO X for a truly nice-sounding and cozy pair of open-back headphones. The DT 900 PRO X provides a more accurate bass representation than the Sennheiser HD 660S. Some may prefer the oval-shaped ear pads of the HD 660S compared to the circular pads on the DT 900 PRO X, but that’s dependent on your head and ear shape.

Save yourself some bucks with the Drop x Sennheiser HD 6XX, which has the signature Sennheiser sound. The build is a bit plasticky, but it has a decent fit and finish, with replaceable cables and ear pads.

The HiFiMan Sundara is where to look if you want to get into the world of planar magnetic headphones without shelling out a king’s ransom. These headphones are comfortable, straightforward, and excellent performers out of the box.

If you want headphones suitable for all types of audio production, the DT 700 PRO X will make you smile. It fits well and sounds good, and audiophiles will enjoy the neutral frequency response and low distortion.

No, because we know burn-in is a myth, we can only determine that any improved performance with use refers to the headset stretching and molding over time to fit your anatomy better leading to better performance through fit.

Technically, you can mix with basically any headphones so long as you have a comprehensive understanding of the flaws or areas of emphasis or under-emphasis in the frequency response, but it makes it harder to mix well. It can be done, however, it’s worth considering some purpose-built headphones.

The HD 599 has a more pronounced bass output than the HD 660S, and in terms of treble reproduction, the HD 599 reproduces frequencies 2kHz-7kHz a lot louder than the HD 660S. This is good for listeners who don’t want to sacrifice bass output with their open-back cans, but it is a more “colored” sounding headphone.

Ultimately, if you want critical listening headphones, the HD 660S is the one to go for.