I think the DT880 has a more expansive soundstage, better imaging, instrument separation, that sort of thing. I posted this in the HD650 thread a few days ago...

I think they are fantastic complements to each other. The HD650 has richer vocals and a lush, comforting sound, whereas the DT880 is much more engaging, exciting and in my opinion, fun to listen to. The mids on the DT880/600 are actually very good in my opinion, almost on par with HD600s which I think are pretty much on par with HD650s. I could sing the praises of my DT880 all day. I've found no fault with it. The 250ohm's vocals weren't as good and I feared the 600ohm would suffer the same fate. I couldn't have been more wrong. They are just fantastic. But be warned, you might forget your HD650s exist for a while.... 
Both are great, but I find the DT880 to be more exciting.
































