Sorry for all these mini updates, but I feel like talking about the HD-650 I just got in.
From my main (best) setup the sub-bass is MUCH more present and more extended than my 598, 600, 580 and Q701. No doubt. I actually expected it to be the same as the HD-600, but it's not. The sub-bass is as present as my DJ100 (I won't go there!), which makes me very, very happy.
Sennheiser's press release make it seem like the HD 650 is a re-engineered HD 600.
Warning: Marketeering-speak ahead.
The FR graphs make the two look similar, with an added "mid-bass hump" on the HD 650. I haven't heard the '580 or '600, but I'd like to.