These are perhaps the most praised headphones, considered the benchmark by many and hailed as marvelous by some very famous reviewers.
So, how do the Sennheiser HD660S sound, in my opinion? The initial impression is pleasing; they bear the distinct Sennheiser signature, which not everyone enjoys, but personally, I don’t mind it. The bass is present and well-controlled, the soundstage is narrower than some competitors in the same category, but it’s still there, and instrument separation meets expectations for the category. It sounds reasonably detailed, too. As a quick comparison, I’ve been listening to them alongside the Denon D7200 recently, and the HD660S seem more detailed.
All is well until I come across one of my reference tracks for dynamics and low frequencies: Nils Logfren’s Bass&Drum Intro revealed just adequate dynamics but well-controlled bass. Moving on to another bass reference track, Billie Eilish’s Bad Guy. It’s a challenging track for any headphones, but the HD660S seemed to handle it well. However, in a part of the song where I’m familiar with a metallic electronic tinkling flitting between the channels, I only hear a faint crackling on the HD660S, which becomes the tinkling of the recording only when it intensifies. I thought it might be my amp struggling, but I was in mid-gain with volume at 1 o’clock, and there was still plenty of power available, so it couldn’t be the amp’s fault.
Alarm bells went off, I reset everything, and moved on to a selection of binaural recordings that I usually only use as a final confirmation. With the HD660S, I had to use them right away, and unfortunately, what emerged was harsh. It’s worth noting that with binaural recordings, the better the headphone, the less it feels like a recording; however, with any track used, it was clear and unmistakable that it was a recording. My brain was never fooled; the vocals sounded nasal and muffled, the bass controlled but artificially colored—no real drum kit sounds like that. The highs were sharp. Almost no headphones returned such paltry results with those recordings, and truth be told, the lesser sibling, the HD599, performs better in the same situation.
In my opinion, the money asked for these headphones isn’t justified. Sure, you can still enjoy music with them, but I don’t feel comfortable classifying them as either Hi-Fi or My-Fi because the latter must necessarily draw on elements of Hi-Fi, which unfortunately are lacking here.
In conclusion, I believe these headphones have unfortunately had their day. In their price range, the competitors are fierce, and to mention one at random, the Hifiman Ananda, at a similar price point, outperforms them in every aspect.
The direct comparison executed in real time with the Spirit Centauri was ruthless. It’s not about them belonging to different categories because, for example, the Denon D7200, at a similar cost, when directly compared with the Centauri, makes you say, “well, but…” whereas the HD660S come out humiliated. No high-cost headphone should humiliate a Hi-Fi headphone, even if it’s low-cost. There shouldn’t be such a gap. If I cost €1,000, I should sound 10% better than the one costing €500, and if I cost €3,000, I should sound 20/30% better, but the weaker one should try to compete with some weapons. The HD660S lose 6-0 / 6-0, whereas the Ananda loses 6-3 / 6-4.
I believe that today, the Sennheiser HD660S are outdated, and you shouldn’t spend €400 on them.
They’re currently the worst headphones with Hi-Fi claims that I know of.