Update on my neutral-island of sonic experience:
HD 600: pretty much any upgrades aren’t possible without sacrificing at least one thing they do right, bust if they had about 4 dB more around 40 Hz they will be perfect. They are truly special, that in my search of headphones have killed and surpassed every single alternative (regardless of price). These have being judged pretty harshly, but can’t really find more faults other than bass extension and level.
I think that 4 extra dB at 40hz than default is easily achievable on the HD6... series with EQ if that is something you can do, no ? The way I've EQed mines, they output more subbass in the 20-40hz range than the K371 when measured on my head with in-concha mics. No noticeable distortion (I suppose perhaps because of the level I listen at - given the displayed Vrms on my Qudelix 5K I'd be surprised to learn that output ever exceeds 100dB in that region).