I was very glad that I was able to try every headphone I have been wanting to audition, and then some. I was quite disappointed in some of the things I head and quite amazed by others. I'll keep it brief, and I wish I had taken notes... I used my 4G iPod running Rockbox, playing FLAC as a source, into my RSA Hornet for all of the comparing. My major testing tracks were:
Tool - Stinkfist from Aenima
Metallica - The Shortest Straw from And Justice For All
Talking Heads - The Great Curve from Remain In Light [Remastered]
Miles Davis - So What from Kind of Blue [Legacy Edition]
AKG K701: Big soundstage, really nice with instrumental/classical stuff - I would probably get these if I listened to mostly jazz or classical. Too harsh with metal, however - Tool - Stinkfist really broke down on them and made me want to take them off immediately.
AKG K601: A nice set of headphones, but really not that noticeably better than my 10-year old HD580s to make me that excited. Maybe a little more bass, but not any smoother or any more transparency/liquidity. I really wanted to like these more than I did.
AKG K501: I had read a review someone wrote where they actually preferred these to 601/701, but I can't see how that's possible. I had them on for about 10 seconds before the incredibly veiled and synthetic sound made me remove them. I apologize if I hurt the owner's feelings by saying that, but they just were not for me.
Beyerdynamic DT880: Similar to my feelings of the K601. A great can, exceptionally comfortable. I didn't give these as much time as the K601s, however.
Denon AH-D2000: WOW. Wow wow wow wow. I have never heard such forceful bass balanced with a completely liquid and lush mid and high end. There was so little harshness or veiling of the music on these things - everything really sung. They are incredibly FUN to listen to as well - the driving bass made me tap my toes and bob my head. The bass was maybe juuuuust on the flabby end, but the owner (who's name I unfortunately forgot) said they had about 100 hours on them or so, so maybe they will tighten up a bit with time. I can deal with it if the compromise is such lushness. They were incredibly well-built as well - the adjusting notch system was like butter. I'm buying my pair tonight, as I write this actually.
Denon AH-D5000: You know it might be weird to say this, but I preferred the 2000's. The big reason is that when I put the 5000's on after the 2000's, I immediately heard a very unnatural "concert hall" (or maybe "chamber") sound. It really was distracting to me and sounded a bit odd. The bass was definitely a bit tighter than the 2000's but there was also MORE of it, which wasn't exactly something I would say I wanted.

I agree with Aman when he says the modded 5000 kind of killed what I enjoyed about the Denons - the sense of energy and fun. It makes me wary to open up the 2000's - I suppose I could try one small mod at a time to tighten up the bass. I don't know the extent of the mods the owner performed, but I have to say I think it was a bit overkill.
These were on other people's equipment and sources:
AKG K1000: My first time using the earspeakers, and it was quite a nice experience. Smooth as glass and handled metal with the same ease as organ music.
Stax SR-007: Another brief encounter I should have made last longer. These things can really sing and have absolutely zero harshness.
Interesting sidenote:
One guy (who's name I forgot) had a setup with his nearly brand-new seeming HD580s he said he got on eBay. These things must have had very low use because they felt like new to me. They had the HD650 cable like mine, as well. However they sounded VERY different - his pair had a lot more bass but also a bit harsher treble. My ten-year old pair lacked the bass oomph of his but also had a bit smoother high-end. As the biggest weakness of my 580's is the lack of energy, I was VERY amazed that his pair sounded so different (he agreed). The only physical difference was that he had put HD600 grills on his, but I've heard HD600s and a/b'ed them with my 580s and they sounded nowhere near as different as these two pairs of 580s.
Overall:
The meet was great, I was happy to see so many nice and generous people. I was absolutely floored by the Denon 2000's and thought they exhibited much of the characteristics of the MUCH more expensive SR-007's - smooth and liquid, but with a bit flabby bass - but for $3750 less I'll deal.
Thanks so much for everyone bringing their gear and spending the time to meet up! I wish I could have brought more interesting stuff!