I feel like I am going to be the first person to actually not being really convinced by the HD620. I used them for 4 days, but just returned them. It is a good headphone, but maybe not the headphone that I need right now. My other headphones right now are Beyerdynamic DT770 Pro X and Hifiman Sundara.
I was listening a lot to the DT770 Pro X recently and became accustomed to them. The brightness reduced a little or I got used.
Comparing to the DT770 Pro X:
Beyers are brighter, more airy and analytical. A little bit better soundstage, but both headphones are great for closed backs.
HD620 is much more relaxed and never bright or analytical. In some songs it has a little sparcle which I liked.
In the beginning I used a (lunashop) balanced cable to be able to easily compare to the DT770 (by connecting both headphones to my amp). I read in this thread that the lunashop cables don't sound well, so maybe that is what I also observed. (baffled). The HD620 sounded a bit boomy on some tracks, which I did not like at all. I did not
really observe it much when switching to the original cable. BUT the HD620 sometimes sounded a little bit "boxy" / "boxed in" to me - especially when I switched to the DT770. The HD620 sounds overall darker than the DT770 and this was putting me off a little bit. I also watched a movie with them and did a comparison of several scenes switching between Sennheiser and Beyerdynamic. I enjoyed the HD620 during the movie, but with the DT770 you feel like you are in the scenes compared to the Sennheiser feeling of sitting in a Cinema.
I read multiple times that the HD620 sounds very natural, but I would actually descibe the DT770 pro X to be more natural. Not sure if that is just my personal preference. The HD620 has that "darkness" which I perceived as unnatural/less natural.
I do agree though that the Beyers are probably a bit fatiguing for some and the Sennheiser is probably not fatiguing to anyone. I really enjoyed rock/metal music with them, but I also do with the Beyers.
In terms of comfort I really like both. No issues with my (small size) head and glasses, but Beyer was a bit better. They are less isolating though, so the Sennheiser feels like a real closed back, which I consider a big plus. Isolation in and out is excellent on the Sennheiser. I might get them again in the future, but right now I think I am looking for something else.