From the Beyer offerings, DT990 is the closest competition to HD650 IMO, not DT880. DT880 is technically accomplished enough, although it's certainly not too far off. I think HD650 outperforms DT990 as well - the Senns are simply the more accurate, more natural sounding headphones - but DT990 puts up a very good fight and wins in some areas. But HD650 stand on their own for me when it comes to musicality. They are just beautiful, beautiful sounding headphones - so darn smooth its unbelievable.
T1 blows the HD650 away.... but obviously not a fair fight. I liked the DT880s but eventually sold them not being able to offer the bass I really wanted. I contemplated trying the DT990s but figured why ***** around .... stair-stepping my way up..... I know I'll end up there eventually. So I got a good deal on a barely used T1. Everything you don't like about the DT880s , etc are rectified. Wonderful phones.