I also was at the meet, and had my mad dogs with alpha pads with me (I was part of the test group, so I had them at the time). I was able to compare pretty close to A/B, and had the same basic esperience. HE500 is a great and capable headphone, but much brighter and a bit airier than the MD. In the short time listening, I liked the MD a bit better I suspect as much because I was used to it as anything. I also decided I'd like to spend a bit more time with the HE500 to see if I want a pair. Right now, the HD650 is in the top seat of desired open headphones (above the HD800 for my tastes).
I'd think that if you like the Mad Dogs, the next logical update should be the LCD-2.
To me, the LCD-2 had more presence and weight (to the music) than the HE-500. Plus it retains the same smooth treble and improves a bit on soundstage + bass compared to Mad Dogs.
Funny... but I honestly can't shake how similar the Mad Dogs sounded to the LCD-2.