Thanks to Najib for hosting the meet at his place and providing transportation, and thanks to Bill and Wang for bringing so many headphones. It was a very fun meet (and for me a welcome break from three days of non-stop work), and for four people, there was quite a lot of gear present. My impressions:
Senn HD650: The first time I heard this can was in Lyric Hi-Fi in Manhattan. It was hooked up to some French player and an Antique Soundlabs OTL 32. I didn't like it at all, greatly preferring the HD600. Well, after this meet, I still don't like it. Hearing it out of my amp made it better, but the "veil" is very present to me and the headphones just weren't interesting to me. A real snoozer.
Senn HD595: Better than the HD650, IMO. Less veiled, more engaging, maybe even more musical. I would take this over the 650 in a heartbeat, but I wish there would have been a HD600 to compare it to.
AKG K701: Kicks the pants off of the HD650, IMO. Very comfy, expansive soundstage, wonderful for jazz and classical. I very much liked these cans, but not more than my W5000.
AKG K501: Wow, these really impressed me with their smoothness and liquidity when listening to jazz. For the price, these are very hard to beat. I might pick up a pair at some point.
AKG K340: Very interesting, but must be driven to high volumes for the midrange to show up. Looking back, I wish I had listened to them more.
I completely missed the DT880, also. I kept telling myself I would try them later but never got around to it.
So, I learned I'm definitely not a Sennheiser guy, and while I liked the AKGs, my W5000 are my favorite out of the bunch for an all-rounder. Now I just need a new source.