It is fairly subtle but did notice some difference.The grill swaps may play a small role. Baffle swap might also play a small role. Clamp would play a small role. And I guess old pads vs new pads etc.
But doing all of them defo results in a noticeable change.
The 660S driver sounds like a 6 series, but it also has some ... i don't even know what word to use... some sort of "edginess" . Perhaps I have used that word before, and someone else used the term "digital glare"
I tried the 660S driver with an older black paper baffle, and a headband from past which was much looser. With a buffer (itube) in system, it sort of smoothed it over.
It's not unlistenable or bad or anything like that, but it leaves me with similar feelings like ath-ad1000x after longer periods of time vs some other Sennheisers.
E.g 580 (II gen) driver with the new (660 S1/2 baffle), Custom Cans copper ring, older model pads, and the 660S chassis (more clamp, new cable), and it's smoother and less fatiguing.
I know the newer drivers might excell in the bass and treble regions vs the older stock versions (580/600/650), but where the mids sit, it might be personal preference.
There are differences in where the mids sit, the mid bass hump levels, and fatigue factors.
Again, it's all relative as I don't think the 6 series overall are fatiguing per se.
I will say, you were quite right about that r70x. It shocked me how much more natural sounding it is vs its ad1kx/2kx brothers,
There is a bit of something off about the HD 660S, a bit of digital glare or something, I also find the HD 660S2 has something off to it that's even harder to put my finger on. I'm beginning to think it may be related to the unsymmetrical venting on the HD 660S/S2, because honestly every headphone I've owned with it bothered me in some way that is subtle and hard to put my finger on. It's something about how they present sound. I also find the HD 600 and HD 650 do seem to scale more than the HD 660S and even the HD 660S2. With the HD 650 I can pinpoint what makes it troublesome to me, it's the bass presentation and slower decay of that headphone, but it may be fixable with some tweaks or on the right system. The HD 600 simply needs some taming of the upper mids and lower treble. The HD 600's rolled off low-end never really bothered me honestly.
The R70x I don't have any desire to really change any part of it's sound response, it's where I want it as is, but like I thought it's stock cable needs to be replaced and it's a harder headphone to drive than the HD 600/650 but not inferior, I find it scales just as much. Honestly back to Beyers though, just got a DT 900 Pro X and quite happy with it, basically it's what I wanted the HD 560S/400 Pro to be. The Senn HD 560S/400 Pro were disappointments to me. Thinking of getting that 100th anniversary DT 770 (with DT 700/900 Pro X drivers).
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