A common trope that I saw elsewhere on the internet before I bought a pair of HD-6XX's, that I have experienced as an HD-6XX owner, is that the 6xx/650 "scales well with equipment." With better and better input devices to drive them with, I have had better and better experiences with my 6XX, more so than with any of my other headsets (my Sennheiser HD1's, Fostex TR-X00's, HiFiMan HE-4XX's, B&W P3 Series II's, and my Audeze LCD-2C's all do not see as much "benefit" from stepping up my input power capabilities as the 6XX has/did).
For Christmas my wife got me a 3rd party (Periapt) Balanced XLR connection for my HD-6XX's, which allowed me to use the (much more powerful/capable) balanced XLR output on my Schiit Jotunheim; the end result made the 6XX's a much more complete/overall headset for me, that stacks up very well in pretty much every way with pretty much every other headset I own. Prior to having access to that kind of cable/input, when driving them off the (much less powerful) single-ended connection on the Jot, the 6XX's were my favorite/go-to for Jazz/Accappella/Singer-Songwriter/Female Vocals music, but they were an afterthought for harder rock. I still don't turn to them for hip-hop or electronic, because I own a pair of Audeze's, but in my experience they really do get better and better with better driving capability.