Sennheiser HD 660S2 review: Wired for sound

Open-backed headphones aren’t for everybody – they leak sound in a most antisocial manner. And wired headphones aren’t for everyone, either – especially not when the wire in question is almost two metres long. But for solo domestic listening, open-backed wired over-ear headphones make a lot of sense – and the Sennheiser HD 660S2 are more sensible than most.

As far as unity of presentation, tonal commonality, detail retrieval and outright presence are concerned, the HD 660S2 have no lessons to learn. They’re a big, authoritative and profoundly informative listen, able to organise even the most complex recordings to the point that each individual strand is simple to follow.

If they were able to summon the same amount of positivity to their treble response as they do to the rest of the frequency range, and if they didn’t look down their nose quite so obviously when confronted with less capable source equipment, they’d be nigh-on ideal. As it stands, though, the Sennheiser HD 600S2 are a definite option.

Sennheiser HD 660S2 review: What do you get for the money?

£449 put Sennheiser’s way in exchange for a pair of HD 660S2 buys a pair of high-performance, high-resolution headphones. If you’re looking for extended functionality of the kind we’ve all become used to from over-ear headphones, the singularity of purpose the HD 660S2 represent may not be for you. If, however, you’re after a pair of headphones that can do the sonic business with very little compromise, read on…

In terms of design, well, this is a pair of over-ear headphones – and is readily identifiable as such. Sennheiser’s not the sort of company to play fast and loose with the look of its products, after all – so the HD 660S2 aren’t about to shock you where appearances are concerned.

The construction is mostly of high-quality, reasonably tactile plastic, with some perforated metal covering the outside of the open earcups and some soft, gently padded velour covering the earpads on the inside. The inside of the headband features some more padding, this time covered in a silkier nylon-like material, and the sliding headband adjuster is metal too. The minimal number of materials, and their quality, make for a coherent look – even the Sennheiser branding on the headband and the earcups is confidently low-profile.

The hanger arrangement is good, so despite a fair amount of clamping force the 260g HD 660S2 are easy to position comfortably and will stay that way for hours on end. The velour earpads resist returning your own body heat for a good while, too, which helps where comfort is concerned.

Sennheiser provides a couple of lengths of cable in the packaging. Each earcup connects via the company’s proprietary two-pin connector, and at the other end of the 1.8m lengths of cable there’s either a 6.3mm unbalanced termination or a 4.4mm balanced equivalent. There’s also a rather ungainly 6.3mm to 3.5mm adapter in the box, which is useful but does lengthen the cable to which it attaches to over 2m.

Once your cable of choice is plugged into your source player, sound is delivered to your ears via a pair of 38mm full-range dynamic drivers. A very light aluminium voice coil intends to help with the speed of impulse response, and a vented magnet system works to reduce distortion. Sennheiser reckons the HD 660S2 frequency response is a startling 8Hz-42kHz – and suggests that by doubling the sound pressure at the bottom of the frequency range, it’s able to provide super-deep bass response while still maintaining the precision and accuracy that’s one of the mission statements of these headphones.

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Sennheiser HD 660S2 review: What did we like about them?

There’s certainly no arguing with the sort of low-frequency presence and solidity the HD 660S2 are capable of delivering. Despite their open-backed configuration, they generate impressively robust punch during a listen to Floating Points’ Anasickmodular – and it’s complemented by straight-edged precision where the attack of bass sounds is concerned. So momentum isn’t compromised by heft, and rhythmic expression is assured. The low frequencies here are a robust underpinning for everything that’s going on above them and controlled with sufficient authority to keep the midrange free from bottom-end interference.

That midrange reproduction may be the single most impressive aspect of the overall performance, in fact. Thanks in part to their open-backed layout, the HD 660S2 conjure a wide-open, coherently defined soundstage, within which there’s more than enough room for each element of a recording to spread out and do its thing. And it’s singers that benefit most of all from the spaciousness of the Sennheiser presentation – they get all the space they need to fully express themselves, and the headphones’ facility with fine detail retrieval and transient response means no scrap of information, no matter how minor or how fleeting, goes astray.

The transition from low-end to midrange and on to the top of the frequency range is level and even, and there’s more than enough dynamic headroom available to make the distance between very quiet and extremely loud in a piece of music very considerable indeed. The facility the HD 660S2 demonstrate with detail retrieval means that low-level dynamic variations are given full expression too, and the consistent neutrality of the tonal signature of the low frequencies and the midrange makes for a unified, singular presentation.

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Sennheiser HD 660S2 review: What could be improved?

In absolute terms, the Sennheiser HD 660S2 are just a little bit sniffy about the source of music into which they’re plugged. If you connect them directly to a laptop, say, they’re not about to soft-pedal your computer’s shortcomings where digital-to-analogue conversion is concerned – they sound muddy and edgy, and not at all like £499-worth of headphones.

If, however, you connect them to one of our favourite DACs (and you needn’t go mad – a decent sub-£150 device will do the job) before they’re connected to the laptop it’s a different story. So bear in mind that the HD 660S2 aren’t about to make a silk purse out of a sow’s ear of a source machine.

No matter the way they’re configured, though, there’s no getting away from the slightly curtailed treble response. Unlike the rest of the frequency range, which is positive and quite upfront, the top of the frequency range is rather circumspect and rolled off. There’s not quite the bite or sparkle at the top end that the tonality of the rest of the frequency range might have primed you for, and consequently, the sound of the highest frequencies isn’t quite as clean or as attacking as is ideal. The Sennheiser are just as poised at the top end as they are elsewhere, but they don’t have the extension or drive to properly bring table sounds to life.

Sennheiser HD 660S2 review: Should you buy them?

If you’re going to be doing your listening alone – because open-backed headphones like these always leak sound outwards as happily as they direct it inwards – and if you’re going to be using a half-decent source of music, the Sennheiser HD 660S2 deserve proper consideration.

They’re not perfect – that top-end reproduction could do with a metaphorical rocket up it – but they get plenty right. If you want a big, detailed and generally pretty explicit sound from your expensive new headphones, you need to put these on your shortlist.