The following review has been thoughtfully crafted by Mr. Indranil Mitra from Kolkata, a cherished supporter, customer, and friend of The Audio Store. We express our gratitude to Mr. Indranil for generously sharing his personal insights into the Sennheiser HD 660S2 Headphones. It is important to note that this review is entirely unbiased, reflecting Mr.Indranil's genuine experiences and preferences. No benefits or exchanges have influenced the content of this review. His valuable contributions significantly enhance our blog, and we sincerely appreciate his unwavering dedication to the audio enthusiast community.
Writing this review has proved to be quite difficult for me. Because the HD6x0 headphones have been like my babies. They are legendary headphones and have stood the test of time, and form the cornerstones of the industry. What these two headphones do, no other headphones can. But once you live with an hp in the long run, you get to know its flaws and the HD 6x0 does indeed have flaws as well. The HD600 does have a bit hot upper midrange area that comes off as aggressive in poorly mastered tracks, and especially when you fit new pads on them. If you have a revealing chain, this aggressive nature may come out to be fatiguing for some. It did for me.
Coming to the HD650, it has a flavoured tuning, so it is quite amp picky and matters of "synergy" comes into play (specially on solid states). The treble is usually muted and the bass is definitely on the flabby side. Neither of the two headphones are that revealing in the bass department. On tube amps, the HD 6x0 is lifted heavily, enhancing their capabilites all round. But the flaws usually still remain more or less. I never heard the HD 660s but to be honest, I wasn't expecting much from the HD 660s2 given my almost unfair bias towards the 6x0 series. But the moment I plugged in the 660s2 in the Anode Acoustics No.2 Solid State HP Amp and put them on, it was instant love.
the HD 600 and HD 650 just sound out-dated in simple words. HD 660S2 has far better bass than both, from sub-bass to upper bass more in quantity and much more in quality. It is much more thicker sounding than the HD600, making it sound anemic simply. Believe it or not, the HD 660 S2 has even more forward vocals than the 600 even, specially male vocals. So much body, weight and heft to them. HD 600 vocals almost sound distant in comparison. And I am sure you are aware of the fact that the HD 660S2 has a bit of a recession in the upper midrange and THANK GOD for that! It is so pleasant to listen to, even with the harshest of recordings whereas they are piercing and grating on the HD 600 due to its spicy upper midrange. But what's even more surprising is that the treble although has more quantity in the 600, S2's feels more pleasant but tighter and precise sounding with an added sense of bite, even though the treble is a bit rolled off! Its as if someone took the HD 600, fixed its flaws and tightened it up! Switching to the HD 650, I was greeted with the same old friendly sound, but did I prefer it over the HD 660 S2? Honestly, no. The HD 650 just sounds mushier and veiled in comparison. HD 660 S2 just sounds tighter, modern and simply more engaging.
Speaking of resolution, the HD 660 S2 has more resolution in the bass and treble than both the HD 600 and HD 650. In fact it is more resolving than both the HD 600 and HD 650 even in the midrange and I did not think that was even possible. The list doesn't end there; HD 660s2 is SIGNIFICANTLY more dynamic than both the headphones. There is attack and punch to the sound, making it more versatile for the more modern genres. It is also wider sounding than both. There are reports of the HD 660 S2 sounding similar to the 58X and yes, there is a similarity but only so much. HD 660 S2 is leagues ahead of the HD 58x in every single way. Graph chasers (reviewers) have been critical to point out that the HD 660 S2 doesn't match the preference target ideally and thus knocked a few points off it. But what they fail to understand is that there is much more to a headphone's sound than just frequency response. Sennheiser has been proving that concept time and time again with bombastic releases like the IE600 and IE900 which do not follow some pre-determined preference curve and yet sound fantastic. The same is the case with the HD 660 S2. These headphones are made to LISTEN to the music and ENJOY them. They are not advertised as studio headphones with reference grade tuning which one can use for mixing and producing. Let's face it, most recordings out there are crap. Too resolving headphones make the experience gritty. Too reference-y grade headphones also do not help mask the imperfections in them. For bad recordings, you need a touch of "warmth" and added "thickness", tame upper midrange that controls grating electric guitars and such. You need smooth upper treble without any absurd stabbing peaks (like some Hifiman headphones have). The HD 660S2 is just that. But it also has tremendous resolution and some serious technicalities to match it up. So I would urge hesitant potential buyers to trust the engineers of Sennheiser with decades of top shelf experience than some youtube reviewers. It has been a few weeks since owning these headphones and I must say they get better and better the more I listen to them.
I really think Sennheiser made a headphone that combines the good traits of both HD 600 and HD 650 while rejecting their flaws. That is a tremendous achievement.