FWIW, my HD650 has better measured bass distortion vs. the HD600 I tested at the same SPL. Tyll's HD650 measurements also showed ever-so-slightly better THD on the HD650 vs HD600. When I was experimenting, FR changes with/without this thread's front-page mods primarily affected mid-bass but otherwise were relatively slight differences beyond that. Playing around with damping material beyond that, even going so far as leaving the back damping in place and removing all front damping, still doesn't get results quite like the HD600 (i.e. upper-mid/lower-treble slight emphasis), and the housing appears to be about the same if not identical on both models. I could be wrong on that latter point.
I have to assume the drivers are actually slightly different or at least cherry picked in some way.
Another tweak that might work is putting lower desity open-cell foam sheets in the cups after removing the stock back damping. This should help reduce mid-bass while still providing a more subtle effect vs. putting something directly behind the driver. Just a guess, though. Lower-density or less foam directly behind the driver should work too if you want something in-between. With the front-page mods, though, there is occasionally a smidge too much mid-bass, but I usually just enjoy it for what it is. I generally like some extra (measured) bass or mid-bass myself anyway unless the driver inherently has a ton of bass slam, which I usually only find in planars with huge drivers.
