Was following you there until the part of not being able to get high subbass or midbass w/o distortion. If you decrease the mids and treble so that the subbass and midbass are boosted (but not over 0db) and then compensate with volume you can get a ridiculous amount of distortion-free subbass and midbass. That is, I can't hear any distortion. Whether it measures as having more distortion or not I'm not sure but it sounds extremely clean and great to me.
If you meant you can't "boost" the subbass and midbass (i.e. above 0db) without distortion... well, yes. But that doesn't have much to do with the HD600.
I'm not talking about going above 0dB. I'm talking about the inherent distortion that the headphone system has when trying to reproduce low/mid bass at high SPL. It's not as bad as some other headphones to be sure, but it's definitely there. It sounds like the bass becoming more "rounded" rather than maintaining clarity. Try it yourself. Again, I'm talking about fairly high SPLs. Overall the HD600 has excellent and adequate bass extension so long as you don't push it too hard.