I hope this is the appropriate place to post this as I've been using/appreciating the 650s for years now. I used to spend more time around the forum and I still drop in to check for any groundbreaking news (or the possible sale of something I'm considering).
I'm fortunate in that I have a friend (he's a head-fi member) who can afford to buy and experiment with much more expensive gear than I can. Because of him I've been able to spend some time with the LC2, T1 and other high end phones along with some great amps, both SS and tube, balanced and unbalanced.
After having spent time with this higher end gear I've come to the conclusion that with properly driven HD650s there are very few reasons to upgrade even if I had more funds. I liked the LC2s a lot but they were uncomfortable for longer periods, the T1s were also great but I found them a bit too bright and (dare I say) a bit thin sounding.
Maybe I've just become accustomed to the sound of the 650s (balanced ->Matrix Mimi-i -> LD MK-Vll+). But when I spend time comparing much higher priced gear, I come away wondering why people spend so much more for what I consider to be very small improvements in some areas. Other aspects of some of the high end cans aren't as pleasing to me as the 650s. It's not that I can't hear or appreciate subtle differences, it's that I just don't find that the very considerable added expense creates a more enjoyable music listening experience.
I suppose if I was filthy rich I'd buy some Sony R10s which I heard at a Toronto meet as I haven't heard anything as exceptional since. The thing is, they were being played through an amp that cost at least as much as these $5,000 (if you're lucky) headphones! Even if I had that kind of cash I wonder if I'd really end up enjoying the music I love any more than I do at present. I guess I'm going to have to win a lottery to find out...just sayin'