For my first HD800 listen, I first prepared by listening with my HD600 on the Balancing Act (Sat AM) for a minute or two and was really liking the sound. Then I switched to the HD800I was struck by the improvement in soundstage, extension, detail, and speed. The HD800 make the HD600 sound slightly veiled in the same way an RS-1 or Stax Lambda series will do to the HD600 in comparison. But, without the colorations in mids or treble of the RS-1 or Lambda series.
It wasn't an OMG moment like the first time you hear an R10 on the RSA B-52, or an HE90 on an A-10 or ES-2 - but after just a minute or two the R10 starts to sound a little bit like a closed can, and the HD800 keeps chugging along sounding natural and real and "open". The R10 are very fun and dynamic, but not as open.
I'll take the HD800 anyday over an R10, modded Denon, modded Edition 9 or any other closed can simply due to the unfettered uncolored open and transparent sound, regardless of whether my first impressions is more stunning with the other cans at first listen. I listened to more HD800 on more rigs at CanJam Saturday than any other headphone except my HD600 and K1000. I am confident that the HD800 are simply going to grow on me and get better and better with time, while the other dynamic cans will start to sound more flawed over time.