Review: Sennheiser HD660S2 – A headphone from the German brand that broke with tradition

The Sennheiser HD660S2 was a surprise compared to other headphone models from the brand that I have experience with, such as the HD600, HD650, and HD6XX. Comfort remains very similar, but the sound struck me as more balanced, with present bass yet without “muted” treble, resulting in a pleasant presentation most of the time. In this review, I also compare it with the Sennheiser HD600, which is my personal daily-use headphone.

Build quality, comfort, and design of the Sennheiser HD660S2

The build of the Sennheiser HD660S2 differs from the first version of the HD600 [1]. I noticed that the plastic used in the HD660S2’s housing produced some creaking noises at times when I moved my face or put the headphones on—something I did not experience with the HD600.

Comfort remains very similar to what I have experienced across the entire HD6XX series. However, I noticed that the shape of the headband padding is different: it is a single curved piece rather than multiple segmented pads. Although it is difficult to say precisely, the padding felt firmer and stiffer, causing a bit of discomfort after several hours of use.

The ear pads remain very soft and provided a good seal around my ears. I also did not feel my ear touching the foam filter on the inner side of the pads—something I often notice on the right side of the HD600.

The thermal comfort provided by the HD660S2 is good, and during the time I used the headphones—several consecutive hours—I did not feel discomfort. However, this can vary from person to person and depends on the climate where each user lives.

In terms of weight, I found the HD660S2 to be just as light as the HD600, HD650, and HD6XX. Since they all basically use the same chassis, this should not be a surprise to anyone who has tried any of these headphones. All of them weigh around 260 g, and the HD660S2 felt extremely light in my hands.

Aesthetically, seeing the HD660S2 in person, I found the color choices to be very tasteful. I prefer minimalist designs, and the matte black of the main structure combined with copper accents conveyed a sense of finesse to me.

[1] The Sennheiser HD600 has two versions. The first features a granite-like finish and was sold from 1997 until the launch of the second matte black version, sold from 2019 onward.

What accessories come with the HD660S2?

The HD660S2 accessory kit includes a 6.35 mm to 3.5 mm adapter and two 1.8 m cables—one with a 6.35 mm (1/4″) plug and the other with a 4.4 mm connector. In my opinion, this was an excellent choice, as Sennheiser previously included three-meter-long cables, which greatly hindered my use, since I mainly connect the headphones to a desktop headphone amplifier.

Unfortunately, the HD660S2 does not come with a case or storage box. I admit I miss this, as I find it very useful for storing the headphones, protecting them from dust and avoiding pressure on the ear pad foam when using headphone stands that support the headphones by the sides and the top.

How does the Sennheiser HD660S2 sound?

In my perception, the sound of the Sennheiser HD660S2 is very balanced in terms of tonal balance (distribution of bass, mids, and treble). It departs from the brand’s traditional sound signature, which focused more on mids and treble. When compared to the HD600, I was able to notice both advantages and drawbacks in the HD660S2.

The bass of the HD660S2 is undoubtedly the highlight of the headphone. There is greater extension compared to the HD600, allowing for better perception of the timbre of instruments and voices that are more prominent in lower frequencies. But that is not all.

Due to the prominence of mid-bass in the low-frequency region, the bass extends much more than on the HD600, filling the entire soundstage of the music. Everything gains more body, including vocals. However, not everything is perfect, and this characteristic can sometimes be a drawback.

The body of the HD660S2’s bass is impressive and adds immersion to the musical presentation. However, in tracks where the focus is on mids and treble, I noticed that some vocals and string instruments sounded slightly odd. When comparing it to the HD600, I realized that the excess mid-bass ended up affecting the naturalness of the mids, which I will explain further.

The mids of the HD660S2 are slightly recessed, giving a greater sense of spatiality. This was initially hard to notice due to how subtle the recession is. However, after listening to many tracks, I felt a lack of naturalness that I could not immediately identify.

After switching back and forth several times between the HD600 and the HD660S2, I believe I figured out the cause of this lack of midrange naturalness. Combining the more pronounced mid-bass with an above-natural brightness in the upper mids, there were moments when I found vocals to sound unnatural on the HD660S2.

The presence of upper mids helps create a greater sense of clarity in the music—you feel that there is more “air” between sounds, making it easier to hear certain nuances. I noticed that the HD660S2 has increased activity in this region, which makes sense given its stronger mid-bass. However, this ultimately led to a certain lack of naturalness in the sound.

Due to the greater activity in the upper mids—and perhaps even slightly in the treble—I noticed that some sounds were a bit “sharp” on the HD660S2 when compared to the HD600. The added brightness in this region helps with detail retrieval, but it can cause listening fatigue more quickly, depending on the track being played.

Overall, I felt that the HD660S2 delivers greater detail when reproducing higher-frequency sounds, such as cymbals. The headphone’s bass activity allows you to better feel impact and volume in music with strong low-frequency presence. The mids sounded somewhat artificial to me, but still delivered solid performance overall.

Combining the mid-bass and upper mids of the HD660S2 results in a good sense of spatiality and a feeling of a “clean” sound. I did not perceive frequency conflicts that made it difficult to distinguish sounds. Even though the mid-bass spreads through the music, it did not significantly harm the mids.

The sound presentation of the Sennheiser HD660S2 is engaging and offers an excellent level of detail without neglecting the bass, which feels very well placed and does not compromise overall sound definition, blending harmoniously with the mids and treble.

Conclusion

The Sennheiser HD660S2 represents the brand’s ability to implement effective sound tuning, showcasing its more than 80 years of know-how in audio equipment. In addition, the HD660S2 retains the renowned comfort and build quality of the HD6XX series, which I believe ensures excellent long-term durability—just as I have experienced with my HD600.

However, there is no denying that its price of US$ 599 on the Sennheiser online store is high. That said, I recommend the HD660S2 to those who are already familiar with Sennheiser headphones and know what to expect. The headphone’s greatest strength is combining good sound quality with comfort and durability, allowing users to enjoy it for decades without difficulty.

Specifications: Sennheiser HD660S2

• Driver type: Dynamic
• Sensitivity: 104 dB (1 kHz, 1 Vrms)
• Impedance: 300 ohms
• Weight: 260 g