Sennheiser HD 660S2 Over-ear Headphones Review

Posted on 20th April, 2023

Sennheiser HD 660S2 Over-ear Headphones Review

David Price auditions the latest mid-price headphone from this respected German manufacturer…


HD 660 S2 Over Ear Headphones

US$599.95​​​ | S$899​​​​ RRP

Hi-fi headphones took off in the mid-nineteen seventies, when stereo LP records became the norm, and half-decent audio systems became affordable to mainstream buyers. At that time, there were countless pairs of stereo headphones on sale, most of which were heavy and cumbersome closed-back designs that seemed more like exotic implements of torture than anything related to enjoying yourself.

Around this time, Sennheiser began to make its mark on the consumer audio scene by offering an entirely different headphone experience. Products like the HD40 were light, easy to wear, and had great sound thanks to high-quality transducers and an absence of volume and balance controls. The German company’s wares also acquired a reputation for ruggedness and longevity, which set it apart from most rivals. All this proved a recipe for success, and since then, it hasn’t looked back. Indeed, all the brand has really done is hone its original formula and scale it up to more premium price points.

The original HD 600 was a case in point; launched in 1997, it embodied everything about the company but at a price many times that of the old HD 40, in real terms. Although not adorned with fancy styling and exotic materials technology, it sounded great and was robustly made. As with most Sennheisers, it was open-backed and offered a clean, crisp and smooth sound with excellent fine detail retrieval. Although not perfect, it was the sort of product that you bought because it was good enough to see off all but the most expensive cans on sale. 


The HD 660S2 is its latest and greatest incarnation, you might say. Superficially very similar to its twenty-five-year-old parent, it shows how timeless the original was. The company says it has been subtly tweaked to offer more impactful and accurate bass – which was always the HD 600’s weak point. The latter was a little soft and stodgy in the low frequencies, although its innate tunefulness meant that this was far from an unpleasant sensation. Sensitivity is said to have been improved, too, although it wasn’t an especially difficult thing to drive before.

Thanks to the revised outer mesh design on the earcups, airflow around the transducers has been improved, says Sennheiser, and a new and more refined voice coil is specified with a lower moving mass. Quoted nominal impedance remains at 300 ohms, the same as the HD 600 and HD 650. The transducer surround drops the resonant frequency from the 110Hz figure of the original HD 660S, down to 70Hz – moving it below one of the ‘hotspots’ for most rock and pop music. A senior loudspeaker designer once told me that his special trick to make small standmounters sound ‘fat’ was by adding bass lift at precisely 100Hz. Here though, Sennheiser engineers have, in effect, done the reverse.

There’s something intuitively right about the design of the HD 600 series, and that still holds for the 660S2. It’s very well made from high-quality plastics, designed to be strong but pliant enough not to break easily. The plush velour earbuds may not be the last word in style but are far better in practice than the ‘faux leather’ (i.e. vinyl) of its rivals. They’re harder to damage, less likely to crack over time, and less sweat-inducing too. The foams beneath are like German car seats – firm or even vice-like at first, but they get better with use and end up feeling like your favourite pair of old shoes!

The HD 660S2 comes with two 1.8m user-detachable cables that terminate to 6.3mm single-ended stereo and 4.4mm balanced stereo jack plugs, respectively – plus a 6.3mm to 3.5 mm adaptor. Detachable cables were one of the great features of the original HD 600, because a flourishing market for audiophile upgrade cables soon appeared – some of which were more expensive than the headphones themselves. This might seem silly, but it means that owners will have an upgrade path if they wish to use it when they’re ready.

Like the HD 600 – and indeed the HD 40, which was my first pair of ‘real hi-fi’ headphones forty-five years ago – one of the hidden delights of the HD 660S2 is living with it. The product comes in an unassuming black fabric storage bag for when not in use. When in use, this pair of headphones is simplicity itself to spend time with, as there are no brittle, over-complicated headband arrangements or zany adjustment systems, and the cable management is great even if the leads are slightly microphonic. Oh, and once the HD 660S2 is on your head, it’s not going to come off unless you want it to. My well-campaigned twenty-year-old pair of HD 600s feel more comfortable than the box-fresh review pair of HD 660s, but I remember them being a similarly snug fit when they were new.


Earlier incarnations of the HD 600 series were excellent all-rounders, albeit flawed. I’ve done a number of headphone group tests over the years, and the Sennheisers always came at or very near the top. Often not the most striking performer, they nonetheless made fewer mistakes than the competition, winning out for sheer overall ability. Little has changed with this latest HD 660S2; it’s still predictably accomplished in every facet. Yet the one bugbear that earlier versions had – that of a slightly spongey, marshmallowy upper bass, has now gone. This new model sounds leaner and more insightful. It’s as if it’s downed an expresso coffee or two and taken a deep breath of fresh mountain air. 

In practice, this makes for a fast, lithe, open and detailed sounding headphone that’s lots of fun regardless of the programme material. I tried it with a range of different types of music, and when powered by the excellent full Class A Musical Fidelity M1 HPA headphone amp, it seemed to have fun with everything it was asked to reproduce. The tonal balance is close to perfect, now that there’s less of a sweet spot in the upper bass. Being more nimble, the sub-bass work on electronic dance music such as the Grooverider remix of Isotonik’s Different Strokes, was a joy; bouncy and tactile with excellent timing, the HD 660S2 really got into the groove. 

At the same time, the midband now seems better focused and more revealing. Gone is the original HD 600’s slight opacity in the presence region, and things are more direct and upfront without ever becoming harsh. The opening piano break on the aforementioned house track was lovely; rich and sonorous with an excellent sense of timbre. It seemed more delicate than earlier 600s I’ve heard too; not in the sense of it being fragile, but rather of it being subtle. I heard the same too when playing the Pet Shop Boys’ Love Comes Quickly from their mid-eighties debut album. The HD 660S2 cut through the mix to tell me what was really going on, skilfully highlighting how the lead synth riff was bouncing around from the left to right channel, and homing in on the electronic percussion. 

All of which makes the HD 660S2 a very revealing instrument, should you wish to find out what’s happening deep down in the mix. Vocal textures are particularly well carried, as the classic rock of Lido by Boz Scaggs revealed. This headphone carried his gutsy voice very capably, unwrapping it for all to see and hear. At the same time, I was struck by the polish of the hi-hat cymbal work; this is a real smoothie of a headphone, yet it doesn’t gloss over treble information, just reproduces it with decorum. And then there was the tuneful, supple and meaty bass guitar that powered the song along – now crisp enough to really sound fast without dominating things further up the frequency range.

So it’s not just fine tonality that marks out this design as special, it’s rhythmic and dynamic alacrity too. Even when playing very sedate music, I was really drawn into the mood, witnessing its handling of The Cocteau Twins’ Lazy Calm, which is an ethereal – almost magical – sounding slice of mid-eighties ambient. The beat isn’t delivered by drums, but by the phrasing of the heavily processed guitar work. Yet the HD 660S2 carried the music in a gorgeous way; the sound was delicate and subtle, yet quite mesmerising with all the different strands of the mix playing along together with great empathy for one another. The result was a blissfully dreamy yet emotionally affecting listen. 

The HD 660S2 isn’t perfect, of course – no pair of headphones is - but for the money, it’s hard to argue with. Perhaps it could have a touch more sparkle in the high treble to give a better sheen to hi-hat cymbals, for example, and maybe it’s possible to improve on this headphone’s sense of space and depth. This Sennheiser images well with an expansive feel, but it’s still not the most cavernous sound around if we’re being picky. All the same, you need to spend a fair bit more to really surpass it sonically.


Another great pair of headphones from Sennheiser, the HD 660S2 ticks all the boxes in terms of sound quality, build and overall ease of use. It may not be the world’s most dramatically styled design, or the most lavishly finished, but its sheer all round quality more than makes up for this. Better still, it’s positioned at an affordable price point that makes it hard to justify the existence of headphones with silly prices.

For more information visit Sennheiser

    David Price's avatar

    David Price

    David started his career in 1993 writing for Hi-Fi World and went on to edit the magazine for nearly a decade. He was then made Editor of Hi-Fi Choice and continued to freelance for it and Hi-Fi News until becoming StereoNET’s Editor-in-Chief.

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