Thanks guys, you've given me a wealth of information right off the bat. Clearly, the Audio Technica ATH-AD900 fits the bill best, with both the Alessandro MS-1i and AD700 as the best lower cost alternatives.
Are we sure that the Beyerdynamic DT880 32ohm wouldn't be easily driven? Their marketing literature advertises their compatibility with portable sources, and their specs seem to suggest that they'd do just fine. I can't seem to find much info on them, even here on head-fi. It seems that the most experienced ears here have no need for easily driven hi-fi headphones, so the 32ohm Beyers don't get any listen time, especially since they're overshadowed by the 600ohm.