I've looked at a lot of freq responses on a number of sites re: HD-600 in particular it was the material between the ear and the driver. You can find fabric (inc toilet paper) that reduce the highs 3-5-7 db, but they also bring up the bass (but its not clean, it's less damped, it's interesting stuff.
It ties in to the 600 v 650. The 650 has more bass, it's less damped and less clean. So you get a choice 600: 95/100 in terms of bass speed/clarity, but 82 in terms of quantity. The 650 is an 88/100 in terms of speed, but a 91 in terms of quantity. But wait: the 650 rise in the bass is an ersatz increase in quantity since the quality is cut.
In addition, there is a guaze on parts of the treble of the 650 - also ersatz IMO. I always had a poor opinion of the 650 - now its more clear why. The 650 is newer, warmer, bassier. The 600 is older, but truer.