Audio Performance

The 38-millimeter speaker drivers built into the Sennheiser HD 660 S are supposedly based on the ones the company uses with their HD 700 headphones, but with a different implementation. As a result, the sound performance of the HD 660 S isn't identical to anything you might get from any other pair of Sennheiser headphones. If you happen to be a headphone aficionado, it's very likely that you're familiar with the so-called "Sennheiser veil", one of the most discussed issues (or myths, depending on who you ask) of their HD 650 headphones; some users consider its treble and upper mid-range to be too laid back, which supposedly results in a lack of clarity and openness. I for one never experienced such issues with the HD 650 and thought it sounded pretty great across the board.

With that being said, the Sennheiser HD 660 S is definitely a beast of its own. The sound quality of these headphones is absolutely amazing. I was blown away from practically the moment I first heard them, and I'm still getting goosebumps on a daily basis while listening to music on them. They are by no means the most expensive hi-fi headphones I ever listened to or bought, but if someone put a gun to my head and asked me to surrender all my headphones but one, the Sennheiser HD 660 S would be the one I'd keep.


The sound coming out of the HD 660 S is impressively detailed, clean, and rich in all parts of the frequency spectrum. The vocals sound delightful and strike a fantastically effortless balance with the instruments. Along with that comes excellent stereo imaging and instrumental separation. What all that means is that the Sennheiser HD 660 S will tackle any music genre with ease. Over the course of the past couple of months, I hit it with classical music, hip-hop, EDM, rock, metal (including its most extreme subgenres), indie folk, MTV pop, and just about any other type of music I could think of. It responded with amazing performance across the board. If there's one thing the HD 660 S does not do particularly well, it's the sub-bass. The specs say that it can go as low as 10 Hz, but that's hardly true. The bass starts to roll off at around 80 Hz, and it's more or less gone at around 35 Hz. Because of that, these headphones won't cause your head to shake in songs that have stuff happening below 60 Hz (think artists like Trentemøller, The Knife, Infected Mushroom, and anyone else who uses heavily synthesized basses). That's not to say that the HD 660 S lacks bass. The bass is present, tight, punchy and controlled with surgical precision. Unless you like your bass overdone and don't mind it ruining the authenticity of what you're listening to, the low-end of the HD 660 S will fully satisfy your needs.

Truly great headphones perform so well that the listener is inclined to constantly turn the volume up, just to get even more of their beautiful sound. I consider this to be what some refer to as "audio nirvana". This effect is very present on the Sennheiser HD 660 S. I pushed all of my amplifiers further than I usually do, at times risking damage to my hearing. I didn't care, though; if it has to go, this would be a great sendoff. Of course, none of that would be possible if the HD 660 S weren't so smooth and non-fatiguing, or if the speaker drivers were prone to distorting when pushed hard.

It didn't take me long to realize that the Sennheiser HD 660 S is my new choice for gaming too. The spatial awareness it provides is mind-boggling; it is a great choice for the most demanding multiplayer shooters. It will serve you equally well for games of other types too, including those that are dialogue-heavy or rely on atmospheric sounds and music to immerse you into their worlds.


Since the Sennheiser HD 660 S is very revealing, you won't get away with poorly recorded music and low-quality DACs and amplifiers. To get the most out of it, it has to be powered properly, with a higher-end integrated sound card, decent portable media player, or smartphone with a good DAC/amp (think LG V20 and better) as a bare minimum. Even though its impedance is only 150 Ω compared to the 300 Ω on the HD 600 and HD 650, the voltage efficiency is only slightly increased (104 dB/V, compared to 103 dB/V), so don't expect to be able to reach very high volumes without a good headphone amplifier.

In my tests, I connected the HD 660 S to the headphone outputs of the Audiolab M-DAC (built-in ESS Sabre32 9018 DAC), NAD C 326BEE stereo amplifier (with the FiiO E10K Olympus 2 doing the D/A conversion), NAD D 3020 v2 integrated amplifier (built-in 24-bit/192kHz DAC), Denon PMA-60 integrated amplifier (built-in 32-bit/384kHz DAC), FiiO X5 1st Gen portable media player (built-in Texas Instruments PCM1792A DAC), and iPod Touch 5th Gen. I also listened to them while plugged directly into the headphone jack of the FiiO E10K, as well as the front audio header of the ASUS MAXIMUS IX CODE motherboard because that one is powered by the ESS Sabre 9023P DAC—just to get a sense of how the HD 660 S performs in a less-than-ideal environment. The portable devices (FiiO X5 1st Gen, and iPod Touch 5th Gen) and "budget" electronics (FiiO E10K and MAXIMUS IX CODE's integrated solution) provided decent sound quality, but weren't able to offer enough power to achieve very high volume levels. However, all of the aforementioned higher-grade amps and DAC/amp combos paired beautifully with the Sennheiser HD 660 S.


Let's start with one of the heavy hitters. "Knights of Cydonia" is an epic rock piece, written and performed by Muse in an attempt to—quoting them—sum up 40 years of rock history in six minutes. From the very first second, where you can hear a strange, alien-like noise and galloping horses (they took a lot of inspiration from spaghetti westerns and Doctor Who, apparently), followed by the opening guitar riff and bass drum roll, you'll know you're in for a treat. Every instrument sounds powerful and can easily be distinguished to where you can narrow your focus down to any of them and just listen to how it's integrated into the song. At the same time, the song as a whole flows beautifully. There's no chance you'll miss the details that might otherwise go unnoticed, such as the slide guitar that's playing just before the first bridge or the complex drum break during the bridge. At around the three-minute mark, one of the best buildups in recent rock history starts. Here, the Sennheiser HD 660 S first demonstrates its ability to wonderfully interpret multilayered vocals of different ranges (Muse's singer, Matt Bellamy, performs all of them). After all of the instruments come together and head towards an epic climax, with what's possibly the Muse's most recognizable staple riff, you'll be hard-pressed to not play a bit of air guitar or drums. All the instruments and vocals continue sounding clean and powerful, be it the violent electric guitars, punchy kick drums or crisp cymbals. The ending of this song is a great test of the level of excitement your headphones (or speakers) are able to provide. If it doesn't move you, you need to upgrade.


Wilco's "Jesus, etc." is a song with a wonderful flow and a whole bunch of interesting details; one is the bass line at the very start of the song. The Sennheiser HD 660 S does a fantastic job of interpreting it and keeping it easily hearable, even when the string instruments join in. Jeff Tweedy's voice can sound annoyingly shouty on headphones that don't have a smooth upper mid-range and treble. On the HD 660 S, some slight shoutiness can be heard, but it never pierces the ears or gets in the way of the general warmth of this piece.


Avishai Cohen's "El Hatzipor" can give you a good idea about the naturalness and dynamics of whatever you're listening to your music with. On the Sennheiser HD 660 S, the piano sounds warm and weighty, and its interplay with Cohen's voice stirs up a lot of emotion, even if you don't understand the lyrics, which is just like the artist intended. At a certain point in the song, several people first start snapping their fingers and then clapping their hands. The HD 660 S does an amazing job of revealing this transformation and making it sound natural, organic. Lesser headphones won't give you an interpretation as detailed as this; you probably won't even realize the finger snapping changed to clapping, or that there are parts when both is happening at once. After the contrabass and the drum join in and the song picks up in pace, the HD 660 S makes sure everything stays cohesive.


Trentemøller's song "Evil Dub" is where the Sennheiser HD 660 S shows its only weakness—a lack of sub-bass. You'll definitely get plenty of snappiness and tightness in the dense, atmospheric bass track, but without the recognizable sub-bass rumble, one you feel more than hear. The same sub-bass region goes amiss in other songs that feature it heavily, such as Daniel Deluxe's "Star Eater", The Knife's "Silent Shout", or Infected Mushroom's "Mambacore".


A somewhat lacking handling of the sub-bass region (anything below 60 Hz) shouldn't be confused with a general lack of bass or the ability to create rumble between your ears. Let's take IconAclass's "Long Haul" as an example. It consists of a massive, bass-heavy beat, a couple of repeating samples and MC Dälek's ruthless rapping. When listened to on the Sennheiser HD 660 S, the song sounds deep, thick, and darkly powerful. Since its beat obviously stays above 60 Hz in the low-end at all times, Sennheiser's headphones will deliver you the full experience. I had the pleasure of seeing MC Dälek perform live, so it was easy for me to hear that the HD 660 S adds no artificiality to his voice at all. When listened to through these headphones, he sounds the same as he did when I was standing three feet away from him.