The original Sennheiser HD600 was released in 1997, and much of what the brand has done since has been a refinement and improvement of that monumental flagship headphone. 20 years later, the most recent refinement is the HD660S. Now with 25 years of history, is the HD600 series relevant in the current headphone landscape, and does the HD660S still hold up?
Build and Design
The HD660S takes its main design cues from the HD600, but opts for a simpler flat black finish rather than the somewhat glossier look of HD600 and HD650. Despite being mostly plastic, the build has a solid feel to it. There’s a moderately strong clamp, but the soft velour earpads do a good job of managing it, and provide a comfortable fit.
The package is excellent, giving you a more upscale unboxing experience than you get with the average set of $500 HiFi headphones and gives you 6.3mm and 4.4mm cables in the box, along with a 3.5mm adapter for the 6.3mm cable. Like many of Sennheiser’s current audiophile headphones, HD660S is made in Ireland.
Despite being dynamic driver headphones, the HD600 series has never been easy to drive. The HD600 was 300 ohm impedance and had 97dB sensitivity, but the HD660S is 150 ohm impedance (for impedance a lower number means easier to drive) and 104dB sensitivity (for sensitivity a higher number means easier to drive). This makes the HD660S much easier to drive than the rest of the HD600 fold.
HD660S gives you excellent detail and transparency, and an overall natural timbre. The sound is more focused on clarity and accuracy than on “fun” or engagement, and provides a great example of Sennheiser’s brand of reference sound.
The bass has a tight, natural presentation, with some rolloff into the subbass. The midbass delivers enough punch to prevent HD660S from feeling “bass lite,” but hip-hop and electronic music in particular will feel a little empty without the low end extension. I did some pad rolling with Dekoni’s selection of Elite pads for the HD600 series, and can offer a strong recommendation for hybrid pads for those looking to fill out the bass a little bit. The change in pads added a noticeable bump in the midbass, along with some subtle improvements to the bass extension.
The midrange is where HD660S really shines, delivering loads of detail, excellent layering and separation, and a really strong natural presentation of instruments and vocals. Vocals are set in the middle of the mix, but can feel slightly pulled back. The treble is somewhat reserved, but extends nicely up into the very top end offering a nice sense of resolution and air.
HD660S has a moderately wide soundstage with good instrument separation and a holographic feeling that’s exceptional for the price. While the imaging does provide generally excellent positioning and weight, it doesn’t feel fully three dimensional and doesn’t strongly present the center of the stereo image in front of the listener.
HD660S presents the opening piano chords of “The Scientist” by Coldplay with all the overtones and complexity – along with just a touch of noise from the piano itself. Chris Martin’s vocals are clear, but slightly nasally (not sure if I can blame the headphones for that), while the guitar, synth, and drums that file in are layered nicely. The overall balance between instruments and the subtle impact in the bass make it sound almost as if this song was mastered with the HD600 family in mind.
A classic rock track like “Barracuda” by Heart also seems perfectly at home with the HD660S. There’s a crispness to the opening chords along with a fiery energy. The bass and guitar blend nicely as the bass doubles the signature guitar riff through the verses, and the bass drum has just enough punch to keep the song engaging. The lead vocal is set back slightly in the mix, but cuts through with the high notes. It’s also the little things that start to stand out, like the acoustic guitar that stays largely in the background, but pokes out in a few places – without a great set of headphones, I’d bet most people wouldn’t even notice that this song had acoustic guitar in it.
Thelonious Monk’s “‘Round Midnight” is a jazz classic, re-recorded by Monk himself a number of times, and covered by a variety of jazz artists. The original recorded version has a loose, improvisational feel: the bass and drums hold down the core structure of the songs – with a nice punch where the upright bass and bass drum sync up – while lilting horns add some punctuation. Often Monk’s work highlights the other instruments, but the piano is front and center here, and HD660S brilliantly captures the piano performance. There’s a great weight and a sense of impact and dynamics to Monk’s playing, and it’s all captured with a lively sense of realism by HD660S.
Comparison: Sennheiser HD650
While there have been a number of entries in Sennheiser’s HD600 series, none stand so tall – outside the original – as the HD650. HD650 was released in 2003, making it nearly as venerable as the HD600 itself. If you’re looking at Sennheiser headphones, you may be a bit confused at why you’d have two very similar headphones in the same series at the same price like HD650 and HD660S are, but we’ll look into the differences to help figure out which one you want.
The first difference you might notice is that the HD660S comes in a bigger (and nicer) box, with a better overall package. While HD650 comes in a plain cardboard box with nothing but the headphones, 6.3mm cable, and 3.5mm adapter, HD660S has a nicer storage box and a 4.4mm balanced cable to go along with the 6.3mm and 3.5mm options. The headphones share a chassis, but HD660S has a different finish and some different accents.
Like the finish, the sound comes down to a matter of taste, but I would actually give a slight edge to the HD650 as a matter of personal preference. HD650 has stronger bass extension and a little more midbass as well, but also provides more presence in the upper mids and treble. The upper mids and treble can be slightly more prone to fatigue in the HD650, but also present the vocals a little more up front and personal. On the other hand, HD660S has a wider soundstage and I perceived aspects of the imaging as being stronger, while HD650 is narrower, but has a stronger sense of center imaging.
Another factor is the impedance and sensitivity mentioned earlier in the review. Despite being the one that includes a 4.4mm balanced cable, HD660S is lower impedance and higher sensitivity, meaning you’ll be able to drive it on a much wider range of devices, while that 4.4mm cable would come in mighty handy for the 300 ohm impedance HD650. While there are a few differences that could go either way, the clear edge for HD660S is in the heightened accessibility and the broader range of devices you can use with it.
The Bottom Line
HD660S maintains the core identity of the HD600 series, providing the natural delivery, strong detail, clarity, and spacious sound Sennheiser’s headphones are known for. It also stakes its own identity in the series by providing a more relaxed, less fatiguing tuning, and a broader accessibility for people who want to enjoy great sound with a simpler setup. While the simplicity of a great sound can certainly be enough – as evidenced by HD600’s staying power – subtle evolutions, like the HD660S, help move the brand forward and remain relevant in the current landscape.