Here's my experience (being a former HD650 owner and current T1 owner):
HD650s...dark, laid back with significant rolled off treble, bloomy bass which at times seems woobly. Nice mids and adequate sound stage. Very uncomfortable. I actually find the LCD-2s more comfortable. To sum it up, I was never a fan...not very inspiring for me.
T1s: neutral/balanced, much deeper and controlled bass, better (more forward mids) and treble that is much more balanced without being strident. Sound stage was a definite cut above the HD650s. Of all the headphones I own/have owned, I find the least amount of faults with the T1s.
With the previous generation of flagships...I would actually put the DT880/600 and HD600s ahead of the HD650s. (And maybe the K701s...I hate agreeing with Acix on this one.)