Well I just grabbed a used pair of HD-6XX off r/AVexchange. They sound awesome through my SMSL iDEA dongle DAC, but I’m not very impressed with their build quality. My Final Sonorous III’s are waaaay better. Are 650s built better than the 6XX? I guess I was just expecting more.
In any case it doesn’t matter that much to me since the sound good. I expect they’ll sound even better when my Schiit Midgard and Modius arrive in the next couple day.
It’s not badly built, just a different material. Hifiman cans use metal parts, yet you can argue that they have worse build quality than Senns.
IMO well-built plastic cans are better than badly built metal cans by a long shot, as it is also lighter as a result, more comfortable for longer use cases.