Sennheiser HD 660S2 open-back cabled headphones – sweet (AV review)

The Sennheiser HD 660S2 open-back is made AVfor sound quality – no noise cancellation, no frippery – just good old sound technology delivered over a 3.5mm or 6.5mm cable.

The world has lost sight of headphones that are purely for listening to music. By that, we mean a completely neutral sound signature – the audiophile standard of a flat (good) response that neither adds nor subtracts from the original music! The only issue here is garbage-in, garbage-out – the better the music quality, the better it sounds. Read How to tell if you have good music (sound signature is the key.

And as we found, a good DAC/Headphone amp can really make these sing.

PS: They are the successor to the HD 660S. The sound is slightly warmer (a bit lower bass frequency).

Australian review: Sennheiser HD 660S2

WebsiteProduct page and Quick Guide
Price$949.95 (free shipping), but shop around. From Sennheiser or Addicted to Audio
Made inIreland
CompanySennheiser (Est 1945) is a German privately-held audio company specialising in designing and producing a wide range of high-fidelity products, including microphones, headphones, telephone accessories, and aviation headsets for personal, professional, and business applications.
Sonova Holding, based in Switzerland – a global provider of medical hearing solutions – now owns its consumer audio business.
MoreCyberShack Sennheiser news and reviews

We use Fail (below expectations), Passable (meets low expectations), Pass (meets expectations), Pass+ (near Exceed but not class-leading) and Exceed (surpasses expectations or is the class leader) against many of the items below. You can click on most images for an enlargement.

First Impression – Pass+

The HD 660S2 are open-back, over-the-ear headphones. That means you expect a wider soundstage than closed back (typically ANC headphones). These focus on sound quality and comfort via a thick headband and replaceable deep foam ear cushions (not memory foam).

The ear cups have an acoustic mesh that exposes the inner speakers. Each cup has a two-pin socket. It comes with a 1.8m 6.5mm unbalanced cable, a 1.8m 4.4mm Pentaconn balanced cable and a 6.5mm female to 3.5mm adapter.

Brief Specs – Sennheiser HD 660S2

  • 38mm Aluminium voice coil Dynamic Drivers
  • Frequency response: 8Hz to 41,500kHz
  • Impedance: 300 Ω
  • SPL at 1kHZ: 104dB at 1V RMS
  • THD: <0.04% (1kHZ, 100dB)
  • Weight: 260g plus cable
  • Carry bag

Comfort – Pass+

They have a surprisingly high clamping force – you know you are wearing them. We expect that will reduce a little over time.

The microfibre cups are deep and comfortable. The headband is very soft. At 260g, they are meant for long listening sessions. We tested them for an 8-hour session – easy.

Build Quality: Pass+

Excellent with a 24-month warranty and replaceable cups.

Sound quality – Exceed

Significant mid-bass is evident from around 40Hz, then it is flat to 6kHz, where it dips a little to avoid harshness and then is flat to 20kHz.

The sound curve is smooth, showing the headphones can handle the input, although the upper treble (15-230kHz) is slightly choppy. Still, it conveys that directionality and airiness you need to be sitting in a concert hall.

As a budding audiophile, I cannot fault the sound. It has more than enough mid-bass to satisfy (perhaps a tad too much), strong mid for vocals and crisp highs for instruments. It is spacious, detailed and has no distorted sound.

Sound stage – Pass+

Open-back should mean a wider sound stage. It is still just outside the ears with excellent left/right separation.

We also test with Dolby Atmos content, which considerably opens the sound stage. It did a fantastic job on 7.1.2 content with all the details and movements.

BTW – open back means everyone can hear your content, so find a quiet room, grab a glass of fine scotch and listen to the music.

CyberShack’s view – Buy, relax and listen to the excellent Sennheiser HD 660S2 open-back cabled headphones

These are close to studio headphones that cost two to three times more. If you love music, have a decent Headphone/DAC amp or know what a balanced input is, these are highly recommended. Oh, and get a high-res 24-bit streaming audio account.

If you are a heathen who likes Bluetooth cans, these are not for you.

Rating Explanation

  • Features: 95 It is a cabled, studio quality, open-back headphone
  • Value: 90 – Considering I have not heard better sound from headphones costing 2-3 times as much, they are reasonably priced.
  • Performance: 95 – 95Out of the box, they are neutral, leaving much scope for an EQ.
  • Ease of Use: 90 based on a 3.5 or 6.5mm jack.
  • Design: 95 – superb, replaceable parts and a nice weight

Sennheiser HD 660S2 open-back cabled headphones








Ease of Use





  • Great Bass for those that want it
  • Very comfortable
  • Can’t fault the sound – it is close to studio quality
  • Perfect when mated to an EQ


  • Consider investing in a headphone DAC/Amp
  • 300 Ω can be too high for some consumer devices