http://www.head-fi.org/t/561234/sennheiser-hd-650-mod-compilation
http://apuresound.com/hdsennmod.html
Theoretically, mass loading the driver baffle might tighten up the bass, given it's a relatively large area piece of plastic, but I have not yet figured out a way to do it that won't be messy as such requires some kind of heavy material to stick on it with adhesive. I came across an HD25 mod kit that has something like this but none of it for the HD600, and since I have no left-over Dynamat lying around as I haven't been into car audio in a while, I haven't any material to test it with.