I would have offer another opinion here. I owned the he500 and now own the hd650. They have a different presentation so it depends on what you are after. The he500 have some of the best mids in it's price range, but actually I prefer the mids of the 650. The he500 is technically better but I found myself reaching for the 650 time and time again until I sold the he500.
Please understand that the he500 is a great hp. I really do love the planar sound. But if I were going for an open planar, I'd pick the he400 as it adds better bass response and is very close to the he500. The other issue with the he500 is the weight. It's heavy...at least for me. The 650 is about a third of the weight and infinitely more comfortable.
When people first change to the HE500 it sounds superior. I know mine did. But after 2 months of usage I came back to the HD650. HD650 has (in my opinion) just unbelievable mids. HE500 actually loses there. HE500 has slightly better bass and definitely better treble...but HD650 is where it's at.
Once you've listened to the HE500 for a couple of months and come back to the HD650...you'll notice it. HD650 is better than the Beyerdynamic T1 and HE500 to my ears. Still gonna try out the HD800 though...