+3 to the above. Quite a lot! Like Austin and everyone has said, HD650 responds to every little change and upgrade very well. Despite the praising of the resolution of some of the higher end cans, the 650 can easily out-resolve anything I ever intend to throw at it, including the rest of my gear!
I mention Lyr since that's what my experience in upgrading was, but I've heard great things about the Crack & Crack+Speedball, Valhalla, WA6, and many others. Go with what appeals to you, but do upgrade. E9 is ok, but you can get a lot more out of those HD650s!
I enjoyed reading the above post (didn't want to quote the whole thing). Its nice to hear someone flesh out a little bit of what they mean when they say the HD650 upgrades well. Sometimes, "upgrades well" sounds like "if you're rich, you'll enjoy it more." Yeah, this post shows some fairly expensive steps, but this post read like they were intelligent steps for someone who has already owned the HD650 for five years.
Edited by imackler - 6/22/12 at 10:47am