Sennheiser HD 660S2

Audiophile open over-ear headphones

In a nutshell

Like their predecessors, the Sennheiser HD 660S2 are great headphones that impressed us with first-class sound quality and professional practicality. Sennheiser has added a few percentage points in the low bass range and transparency without compromising the proven balance and definition of the previous models. They thus succeeded in bridging the gap between audiophile sound enjoyment and the provision of the necessary properties for professional “sound engineering”. For first-time buyers, the Sennheiser HD 660S2 get our definite recommendation. For existing customers, a change seems less urgent. After all, Sennheiser has raised the recommended retail price by one hundred euros compared to the equally excellent previous model. In addition, these can currently be purchased at a significant discount.

  • audiophile balanced sound
  • improved reproduction of low bass
  • additional balanced connection cable
  • price rise compared to predecessor model

The Sennheiser HD 660S2 courageously follow in the footsteps of their predecessors and, like the latter, turn out to be a crossover between a tool for pleasurable listening and professional work equipment. In fact, the German manufacturer succeeds in further improving a balanced and wonderfully detailed basic character in the low bass and treble reproduction so that the HD 660S2 earns our recommendation as great over-ear headphones, despite the price increase.

For years, headphones in the “six-series” from Sennheiser have provided a guarantee of superior sound quality with a convincing price-performance ratio. Now, the HD 660S2 replaces the previous top model in the range, the HD 660S from 2017 and its revision from 2019.

With a retail price of over 500 euros, one may be forgiven for having high expectations. In my opinion, the previous models lived up to these, with the HD 660S being an excellent choice for discerning listeners looking for an open, dynamic over-ear system under the one-thousand euro mark, as well as for professionals who need a reliable working tool for production and editing work.

Sennheiser HD 660S2 – differences from previous models

The new version looks virtually identical except for the red and gold logo. The smart black design with elliptical forward-inclined, tiltable and slightly rotatable earcups with velour padding, protected on the outside by perforated grilles, remains. Sennheiser also continues to rely on silicone padding for the adjustable headband.


Of course, the connection cables, which are routed on both sides, can be exchanged: they are available in two formats, one with a 6.3mm jack plug and one with a symmetrical 4.4mm connection, and they now have a shorter length of 1.8m. An adapter for a 3.5mm jack format is also included. For transport, Sennheiser includes a soft carrying pouch instead of the previous pouch, which offered a little more protection but took up more space.

The main changes are design-related. According to the manufacturer, the airflow behind the 38mm drivers has been optimised, resulting in increased sensitivity. Furthermore, the aluminium voice coil of the driver has been revised, so it now has an impedance of 300 instead of 150 ohms and is explicitly intended to be impulsive. However, the most important change is an adjustment to the frequency response. Looking at the comparison curve on the manufacturer’s page, you will see an increase in the bass and low-mid range up to 500 Hertz as well as at higher values from 6,000 Hertz. In the low bass range, Sennheiser claims a doubling of the level compared to the previous model.


The Sennheiser HD 660S2 in practice

The Sennheiser HD 660S2 are headphones for pure music enjoyment. Thanks to their light weight, good padding and comparatively tight fit, they are comfortable and secure to wear. In addition, they can be operated with a smartphone with a 3.5mm jack connection to a professional headphone amplifier. They also provide access to audiophile solutions via the supplied symmetrical cable. There are no other extras, but in addition to the connection cable, the ear pads are replaceable. The overall workmanship is of high quality and I had no problems with the fit, even during longer listening sessions. The padding is just as impressive as the open construction, which sometimes prevents sweating. At the same time, it should be obvious that the Sennheiser HD 600S2 are not completely silent.

The sound of the Sennheiser HD 660S2

If you look at the frequency response of the dynamic driver (and any other dynamic driver), you will hardly ever see a smooth linear frequency but a curve that may well fluctuate by up to ten decibels across the central listening spectrum. From this, I conclude that, apart from the choice of materials, it is simply a matter of the manufacturer’s expertise in construction and tuning. Sennheiser has done a great job here because the HD 660S2 boast a spacious, airy and extremely detailed sound. The frequency response remains balanced, and the basic character is analytical without being cold. In addition, the HD 600S2 continues to operate with low distortion at high levels.

The Sennheiser HD 660S of the first generation were available for comparison, although a completely neutral sound comparison was not 100% feasible without measuring tools due to the different impedances. At the first listen, Sennheiser have matched the popular sound character of the previous models. But if you listen in more detail, the HD 660S2 actually provides slightly more room for improvement and additional capacity in the low bass. Sennheiser is justifiably proud of this, as typically low bass is audible with more expensive models but is kept rather lean in the tuning so as not to overpower the higher-frequency ranges.

At the same time, the transmission of these signals is definitely desirable because the lowest notes of a grand piano, a five-string bass or a synthesizer can add impressive value to a production and are also part of modern sound styles and genres, such as hip-hop and urban.

In fact, the unit we tested was just as tidy, contoured, dynamic and analytical in the bass range as before. In various productions, the extended bass was hardly noticeable at first. In other cases, however, it certainly was: in the middle section of Yello’s “Pan Blue”, there is a passage with prominent low bass. And it was here that the HD 660S2 gained an audible edge over its predecessor.

The central mids offered unchanged detailed listening pleasure. Pink’s “When I Get There” features an intimate vocal recording with a discreet reverberation chamber and a harmonious piano. Due to the fast response, the doublings in the chorus were perfectly discernible. I noticed similarly great results with “Moonshot” by Myles Kennedy. The voice sounded completely natural, offering the necessary closeness, while the acoustic guitar in the verses added a wonderful sheen and body to the arrangement. Orchestral recordings also revealed a coherent authenticity, while acoustic jazz recordings underpinned the headphones’ dynamic capabilities. In all cases, the Sennheiser HD 660S2 maintained control, and I am pleased to say did not lack the necessary warmth. Even with distorted guitars, the results were consistently coherent. These headphones gave the immensely mid-emphasised mix of Slayer’s “Repentless” enough bite for it not to seem dull and tired.

Finally, the added value was also evident in the transparent range of high frequencies. The Sennheiser HD 660S2 provided exemplary airiness for its price class. In addition to fine details, they delivered brilliance and, due to the impulsive approach, the desired contour in delineating instruments in the room as well as in their dynamics. The HD 660S2 was equally confident when it came to the critical limit of harshness.

Finally, they also earned our praise for their spatiality. The broad stereo panorama and spatial depth of “Celestial Echo” by Boris Blank and Malia were impressively reproduced, turning it into a real audio adventure. The differences from the first generation HD 660S were subtle but audible. The “old” version sounded a touch warmer, not quite as open and not quite as impulsive.

1 year ago by Ulf Kaiser
  • Rating: 4.75
  • Sound
  • Handling
  • Price/Quality
  • Function

Measurement Results

Frequency response:

Exterior noise damping:
More measurement results

Technical specifications

  • Ear couplingOver-ear
  • Typeopen
  • Transducer principledynamic
  • Frequency response (headphones)8 - 41.500 Hz
  • Impedance320,6 ohms
  • Sound pressure level (SPL)99,25 dB
  • Pressure averaged from big and small head653 g
  • Weight with cable311 g
  • Weight without cable256 g
  • Cable length180 cm

What's in the box

  • 6.3 mm cable
  • 4.4 mm cable
  • 6.35mm stereo jack
  • Transportation bag

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