I’m curious about what you normally listen to, and what you have listened to. When I checked your profile, there isn’t any commentary about your equipment, daily driver, etc. And this a thread about Sennheiser. I’d lived with the original HD-580 since about a year after it came out, and got the 6XX from drop. I have not listened to the HD-600, which I suspect may be the best of the lot, or the newer models along similar lines.
I do have a range of DACs and amps, solid state and tube, nothing super TOTL, but generally decent stuff. @generic and @robson are trying to point out that it’s not merely impedance and any impedance-related frequency shifts that count. Nor is EQ a substitute for the beneficial harmonics in the sound of a decent OTL tube setup.
For reference, my at-work (non-speaker) setup consists of the Schiit Modi Multibit, Eufonika H7M OTL tube amp and ZMF Auteur Classic headphones. It’s good enough that I don’t aspire to any higher level setup there. (well possibly the Atrium open…). I don’t think @generic has had the opportunity to audition the Modi Multibit (on LAST CALL!) but I find it an entirely acceptable little brother to the Bifrost which we both have, so I’m guessing that he could happily listen to this setup all day.
The point being, it’s very important to hear with your own ears, and your opinions will be additionally informed.