[Listening test] Sennheiser HD 620S and HD 660S2 headphones, one against the other

Manufacturer: Sennheiser - Germany
Price: HD 620S $299 and HD 660S2 $499 (USD)
Reviewer: M.L. Gneier - TNT-Audio USA
Reviewed: October, 2024

I think you have to be pretty jaded not to appreciate the sheer range of choices when it comes to headphones in 2024. From IEMs to earbuds to conventional wired headphones like the new Sennheiser HD 620S and HD 660S2, there is a lot to choose from, and that's a good thing. Headphones are like loudspeakers. They are the most objectively imperfect audio devices. Just compare the response curve of any loudspeaker (in-room or anechoically) to, say, an amplifier and you'll get what I mean. So, that leads us to the old demon of subjectivity.

To some, subjectivity is a little scary, meaning, as it does, that there's never a convenient right (read: objectively provable) answer. To me, that's one definition of paradise. Face it. Everyone hears differently. Everyone experiences music in their own way. So, just give me enough choices and I will find a pair of headphones that sound musical...to me. Everyone else is on their own.

Like millions of other audiophiles I bought my first pair of Sennheisers when I was just a kid of fourteen. Prior to discovering the now-legendary HD 414 I had become addicted to a small parade of marginal headphones, particularly a pair made by the also long-defunct Olson Electronics. The Olson Model 204 cost $7.98 back in 1974. That cost was roughly three times my hourly rate at my part-time job at Electronic Expediters. I'm not sure but I think the HD 414 cost right around $50 a year or so later at also long-defunct Federated Electronics.

Open-back headphones were not very common back in the 70s. I was initially a little disappointed that the HD 414 offered no isolation but I really appreciated how much longer I could listen to them compared with the suffocating vinyl seal of the Olsons. Bass speed and precision were astonishing and the top end was luscious. I would love to hear a new pair of HD 414. No, not a pair made today, God knows where, with God knows what kinds of parts. No, I'd like to hear a new original pair of HD 414.

The HD 414 absolutely spoiled me for other brands of lesser headphones. Sennheiser become my long, long, long-term reference headphones before a lot of the companies that make headphones today were even founded. My reference is the HD 600. When I tell people this they are often surprised I don't use a higher-priced model like the HD 800S or the HD 820. It's the whole objective versus subjective problem again. I am sure the engineers at Sennheiser can give me some solid data establishing the objective superiority of the firm's new designs but I prefer my old HD 600. How old are my HD600? You know, I'm not really sure. I know I've replaced the ear pads at least four times and the head band cushion at least as many times if not more. By the way, thank you, Sennheiser, for supporting this legendary product with a continual and reliable stream of replacement parts. Please keep it up! Oh yeah, I also replaced the original cord with one from a different Sennheiser model so I could ditch the 6.3 to 3.5 mm adapter and its contact resistance. Plus, my current cable is significantly shorter. I wonder if that voided my warranty?

But this review is about the HD 620S and HD 660S2 so let's discuss those for a bit. Let me get ahead of myself and say that listening to these three headphones was an interesting experience. As a closed-back design the HD 620S sounded the most different in the sense that it strayed boldly if not wisely from what I think of as the classic Sennheiser sound. Apparently, one of the design goals of the HD 620S was a more open sound than might be typically expected from a closed-back design. I must say the overall presentation of the HD 620S strikes me as too forward, especially in the low treble and upper midrange. The Sennheiser website has a graph on the HD 620S page showing an overlay of its response versus the HD 600. My favorite line is: "The frequency response is supposed to be perfectly flat in loudspeakers (i.e., all parts of the music are equally loud), but ear geometry and psychoacoustics mean a good headphone frequency response looks very different. While the science is not yet settled, most engineers and scientists in the headphone industry would agree that the HD 600 with additional bass is closer to a neutral, “true-to-life” sound.

Uh, yeah but what's noteworthy is that the response curve indicates that the HD 620S's low frequency response is actually flatter than the HD 600. See what I mean about the near-futility of the objective-versus-subjective argument?

No matter, the forwardness of the HD 620S's presentation robs the music of its natural sense of acoustics while making pretty much everything sound processed for lack of a better word. Bass was exceptionally solid, as I would expect from a closed-back design. Can I imagine listeners who would enjoy the HD 620S? Of course, but I am not one of them.

Before I discuss the sound of the HD 660S2 I'd like to address the build and ergonomics of both models. Do I prefer their black look to the rather dated fake marble finish of my HD 600? I do, but the overall feel of the newer designs do not strike me as an improvement. The current models seem durable enough but they also feel rather plasticy, significantly more plasticy than my cosmetically-challenged and aged HD 600s. As far as comfort both are marginal. Note in my photo that my old HD 600 headband has four cushions while both of the new models have only two. I cannot understand creating a gap in the center of the headband's padding, it being at the point where the mass of the headphone weight will be focused, the top of the listener's head. I did not measure it but I would say that the clamping force of the HD 620S is greater than that of the HD 660S2. Will it break in and become less uncomfortable? I'm sure it will but I am still dubious about the long-term comfort of the 620S.

I was jazzed to have the opportunity to hear the HD 660S2. Part of me expected to be blown away by an across the board improvement over my loyal HD 600. You know what it's like. You hear about a new design that's an obvious evolution of a design you already respect and you essentially pre-program your brain to be impressed. That's why I did. Starting out on this review made me reflect on the history of my reference pair of HD 600. They have traveled more than I have; home to Germany many, many years ago for a repair I can't even recall. Later, I loaned them to a colleague in Greece and later to another colleague in Singapore. My headphones were transducers I knew I could trust and I wanted my colleagues to enjoy the same advantage when they needed a reference. The HD 660S2 is a good sounding pair of headphones. Far more than the humble 620S they allow me to hear the family sound of Sennheiser and some of the fundamental musicality of the HD 600. But, sadly, not all.

Head to head the HD 660S2 sounded a little more dynamic than the HD600s. In terms of sheer dynamic swing I am sure anyone could hear its superiority. But, you really have to push them to hear what is, in the end, a very slight and wholly subjective advantage over the old HD 600. And, let's face it. I am here lauding a very old version of a headphone that is still in production in some form. Is that unfair? Perhaps. After all, I have no idea how the current version of the HD 600 sound compared to mine or either the 660S2 or the 620S. Still, I am confident in what I hear from these two new designs. No my ears, they do not measure up musically to my HD 600. For me the greatest negative of the two new Sennheiser is the slight but pervasive processed sounding presentation.This effect is especially easy to hear on acoustic music; solo piano, voice and the like. So, if you are considering auditioning either I would very much encourage you to consider the heart of your music collection. Does your music rely on fundamental acoustic truths, what I like to call tone, timber and space? Or, is your music of choice better served by what might be called objective strengths like low measured distortion and flatness of frequency response, to say nothing of raw dynamic range? So, as always, it is up to you. I have told you what I heard from these new designs from Sennheiser, a transducer company I still hold in the highest regard. Now it's your turn to listen for yourself.

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Sennheiser HD 620S and HD 660S2

Family match, new vs old generation