The 650 needs a good amp to really be noteworthy amongst other high end cans. When I had a more meager setup I found myself getting bored with it every so often and impressed with just about anything else over $150 that was colored in some way. It took my Burson (not saying it takes this exclusively - it's just the upgrade I ended up making) for the 650's to sound remarkably better headphones like the SRH 840's, 940's, and even the PRO 750's.
Oh yeah, I feel like I should clarify that my relatively recent acquisition of 3 higher-end open headphones was more about product fetishism than me being bored with the 650's. While the HD800's, HE-500's, and LCD-2's are all better headphones, my original impulse had nothing to do with the 650's being lacking; they're excellent all rounders and I could easily think of at least one or two aspects of them I'd splice into those headphones that are 2 to roughly 5 times as expensive if I could.
Edited by driver 8 - 3/17/12 at 11:00pm