Hardware Reviews

Sennheiser HD660S2: a richer experience

Sennheiser HD660S2 review

Sennheiser HD660S2 headphones

Designing the HD660S2, a mid-priced, dynamic open-back headphone, to supplant the legendary HD600 and HD650 was never going to be easy. When Sennheiser finally made their 600 series into a trio in 2017 with the release of the HD660S it’s fair to say it caused some confusion among HD aficionados. Despite using lower impedance drivers to improve compatibility with voltage-limited sources, many viewed its familiar sonic signature as a missed opportunity to offer something more different. The German headphone manufacturer took this feedback onboard and has now unveiled the HD660S2, a design that – on paper at least – responds to repeated requests for a less up-front tuning with better bass extension.

Aesthetically the HD660S2 is very similar to the HD660S and retains its stealth, matte black look but there are three notable changes. The accents on the grille, yoke and headband are now bronze instead of silver, the Sennheiser headband branding is more discreet and the yokes have been given a flatter profile. Like all 600 series models the S2 provides strong clamping force out of the box, a feature fellow slim-heads will appreciate in a market awash with looser-fitting designs, this headset stays put no matter how vigorously you headbang to your favourite AC/DC track. The clamp does relax over time but broader-browed listeners can hasten the process by extending the headband to its longest setting before gently bending the flat metal sections outwards. Despite the design being almost 30 years old I still rate it amongst the most comfortable on the market at its price and beyond. It weighs just 260 grams and is extremely comfortable to wear for long periods, the plush velour pads distribute pressure evenly around the ears and the relief section notched out of the headband padding prevents the dreaded pressure spot on top of your head.

Sennheiser HD660S2 review

Shrinkflation, for a change

The sturdy presentation boxes that 600 series headphones have shipped in for many years have been discontinued. All models now arrive in a soft, black carry pouch inside a smaller and lighter printed cardboard box that’s more convenient to transport and store but does not deliver as impressive an unboxing experience. While this shrinkflation will inevitably disappoint some there will be others who’ll applaud Sennheiser for taking the necessary steps to keep its pricing competitive. The manufacturer has also recently shortened the length of the cables it bundles with its headphones. The HD660S2 ships with two leads, one is terminated with a 6.3mm (quarter inch) jack and comes with a 6.3mm to 3.5mm adapter and the other has a 4.4mm Pentaconn jack for balanced drive. Both are now 1.8 metres in length having previously been 3 metres. I can only assume this is a response to customer feedback but shorter tethers are most useful for on-the-go listening and high-impedance open-backs aren’t typically selected for this. Most listen to 600 series headphones in static systems and a 1.8m cable struggles to reach an amplifier that isn’t within arm’s reach of the listening seat. The longer 3m cable can, however, be purchased from Sennheiser for a small additional outlay.

The decision to revert to 300Ω, 104dB/1Vrms transducers for the S2 ought to please HD600 and HD650 fans who enjoy exploiting these headphones’ legendary scaling abilities with amplifiers that swing decent voltages. When I reviewed the slightly easier to drive 150Ω HD660S in 2017 I noted it delivered more bang for your buck than its 300Ω siblings with entry-level sources – especially those with limited voltage – but didn’t improve as much with high-end amplification; the 300Ω S2 should behave more like the 600 and 650 in this regard. According to Sennheiser, the HD660S2’s lighter voice coil generates higher magnetic force and electrical damping for a cleaner impulse response and the improved airflow around the transducer increases the efficiency of the magnet gap, the combination of which permits more precision, speed and resolution than its predecessor.

Sennheiser HD660S2 review

The new 38mm driver has been built with a more compliant surround to lower the resonant frequency (Fs) to 70Hz to deliver stronger output at sub-bass frequencies. Sennheiser’s published measurements show an increase of +6dB in the lowest octave over the 660S. Some of this low frequency gain is admittedly due to the way the two headphone measurements have been overlaid – the frequency response curves have been matched to the upper middle and lower treble frequencies where the S2 has significantly less output than the S – but every additional decibel in bass extension is an achievement given the inherent low frequency limitations of open-back designs. The measurements also show a substantially elevated high frequency response in the new model, this is again partly a psychoacoustic effect of the S2’s mellow transition from midrange to treble that makes the frequencies above and below more audible.

HD660S2 sound quality

Notwithstanding the familiar feel of the elliptical velour earpads that tells me I’m listening to a 600 series headphone before even a single bar of music is played, the HD660S2 is by far the most distinctive-sounding model in this line to date. It’s much warmer and smoother than both the 600 and 660S and is very relaxing to listen to. The 650 is also a warm headphone but achieves this through a plumper upper bass and darker top end, it retains the HD600’s strong midrange presence and can still come across a little too forward with some source material. The HD660S2 by comparison is extremely forgiving, its laid-back handling of the midband sweetens even the most acerbic recordings and ensures you’ll never again need to worry about the harshness or glare that might have kept certain areas of your music library off-limits with the other three models.

Sennheiser HD660S2 review

I can see this headphone being the perfect choice for rockers or metalheads who’d like those distorted, double-tracked electric guitars not to be thrust so far forward in the soundstage. When I listen to Whitesnake’s Live In The Still Of The Night on the 600, 650 or 660S, Doug Aldrich and Reb Beach’s relentless wall of sound fatigues my ears after just a few songs. The S2 dials the intensity of the duo’s power riffs down a few notches, creating some much-needed space for the other band members to fill, and finally lets me enjoy this two-hour concert from beginning to end without needing a comfort break!

Rim shots and sibilants

It isn’t only high-energy music that benefits from the HD660S2’s sweeter voicing. Depending on the recording it’s possible for any genre to excite the peaks in a headphone’s frequency response, and those peaks can be bothersome if they coincide with the frequencies the listener is sensitive to. The S2’s docile presence region and smooth but airy top end is much less likely to trigger those with sensitivities in these areas than its siblings. For me, the fervour with which the 600 and 660S reproduce the snare rim shots and vocal sibilants in Sting’s wistful Fields Of Gold jar a little with its otherwise beguilingly flawless production. The HD660S2 delivers these two integral instruments with a calmer energy that better matches the perfectly-judged contributions from the other musicians. There are inevitably going to be recordings that don’t benefit from the S2’s warmer presentation. Right Down The Line (Gerry Rafferty) and Solitary Man (Chris Isaak) are two that already verge on sounding overly thick and laid-back for my preferences and the S2’s downward-sloping transition from lower to upper midrange only emphasises the lack of clarity in these mixes.

Sennheiser HD660S2 review

Though I don’t perceive as much as a +6dB increase in sub-bass extension, this region is noticeably more present on the new model than on the original 660S or indeed the 600 or 650. No matter whether you listen to classical, jazz, pop or EDM, the S2’s ability to dig deeper and deliver the lowest octaves with more weight undoubtedly enhances the engaging sense of realism, you aren’t missing much of the solid foundation that underpins many performances. The S2 is better able to convey the mighty scale of the church organ inSymphony No. 3 in C Minor, Op.78 Organ (Saint-Saens), the richness of the upright bass in So What (Miles Davis), the deep rumble of the synth swells in Right Here Waiting For You (Richard Marx) and the foot-tapping drive of the disco-inspired groove in Giorgio By Moroder (Daft Punk).

Some may be wondering whether the HD660S2 moves the 600 series any closer to the 800 series in its ability to deliver a compellingly out-of-head listening experience. As the S2 places midrange-focused instruments deeper in the soundstage it does sound a little more spacious than its siblings, but its presentation remains on the intimate side. This shouldn’t be a surprise given it uses the same earcup and earpad design that positions the transducers close and parallel to the ears. At three times the price, the cavernously wide and deep soundstage rendered by the HD800S keeps Sennheiser’s flagship open-back in a league of its own, though – in fairness – there are very few headphones on the market that rival it in this area irrespective of cost.

Scaling and synergy

While the HD660S2’s tuning is more forgiving of what’s upstream than its 300Ω siblings, it retains its ability to scale to a level beyond its price when partnered with more capable sources. Significant improvements in dynamics, texturing and layering were heard when stepping up from the single-ended output of an iPod Touch to the balanced output of an Astell & Kern SR25, and also from a decent, and commensurately-priced desktop system comprising a Schiit Audio Bifrost DAC and Jotunheim amp to high-end setups costing many multiples of the Sennheiser.

Sennheiser HD660S2 review

Regardless of price I favoured sources that prioritise speed and incision over fluidity and timbre. Chord’s nimble Hugo 2 DAC/amp proved an excellent pairing, Schiit Audio’s Yggdrasil OG multibit converter with either the M2Tech Marley MkII or Sparkos Labs Aries amplifier was even better. The calmer and more relaxed approaches of Holoaudio’s Bliss and Audio-Technica’s tube-hybrid AT-HA5050H were too much of a good thing with this headphone for my tastes, the stronger leading edge detail delivered by the M2Tech and Sparkos is a better counterpoint to the S2’s mellow voicing. Marley is an especially good choice as it’s not only priced closer to the Sennheiser but also features a defeatable 3-band EQ, crossfeed circuit and adjustable output impedance that provides lots of scope for tweaking. I found the EQ especially useful for injecting more energy into some overly warm and polite mixes, with this facility at hand the S2 became a highly satisfying all-rounder for my varied listening habits.

Sennheiser HD660S2 verdict

The HD660S2 delivers a richer and more refined listening experience than its predecessor with superior bass and smoother, airier treble extension. Its laid-back presentation may not appeal to fans of the bolder, more exciting HD660S but is perfect for those who’ve yearned for a forgiving 600 series headphone with a stronger bottom end. The 660S2 achieves this without sacrificing resolution and is a very welcome addition to Sennheiser’s headphone legendary line of dynamic open-backs.


Type: dynamic, open-back headphones
Ear coupling: over-ear (circumaural)
Transducer principle: dynamic
Driver size: 38mm
Sound pressure level (SPL): 104 dB (1 kHz, 1 Vrms)
Frequency range: 8 Hz to 41.5 kHz
THD: <0.04% (1 kHz, 100 dB)
Adapter: 6.3 mm to 3.5 mm
Connector: 6.3 mm jack plug, 4.4 mm balanced plug
Cable length: 1.8m
Weight: 260g
Warranty: 2 years

Price when tested:
£499 / €599
Manufacturer Details:



Open back headphones


Richard Barclay

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