The pro does have a different flatter flavor, yes. Both sound great so dont let that stop you from buying if that is something you want, its still very enjoyable for rock or pop. As for the bass lol, well both have very impacting and large bass. I wouldnt recommend either to you lol, they are both so large and heavy, why on earth would you want that :)
I would recommend the Denon D1100 if you want some crazy bass. In fact, I would definitely recommend that or the D1001 over the Ultrasones any day. Great sound quality but at the price of looking like a radar dish and having a lead weight on your head isnt worth it.
In terms of sound quality, how do the D1001's compare to the DJ1/HFI-580's while using an iPod as the source?