TL;DR: Heard HD700, want it right now.
So I went to the Verkkokauppa shop in Pirkkala today and found they had what seemed like a hundred headphones to audition. I was like a kiddy in a candy store.
HD800, HD700, HD650, HD598 and continuing downwards. The 700 and 800 were being powered by
something called a "HDVD 800". I've seen pictures of it but know nothing except it costs several months rent. Unfortunately, I went underprepared and only had some random compressed music from my phone, but I listened to the 700 and 800 for about 30 minutes each and was extremely impressed, to say the least.
Most comfortable headphones I have ever put on my head. I like the form factor of the 700 more, they seem to have enough room in the cups for me and don't take up half your face. I couldn't feel any clamp with the 700, and a minimal one with the 800. I have quite small hands and almost put my finger through the silver mesh thing on the 800. Must grip from outer-edge.
Soundwise, you guys know already. No point preaching to the quoir. Best two sounding headphones I've ever heard.
I had the 700 box in my hands at one point. €498 (556USD) is somewhat more than I'd like to pay, though. If money were no object and music was my main concern, the HD800 would win my heart because even someone with untrained ears like me can hear they are somewhat technically superior. It's almost like you're not even listening to music with the HD800, with the HD700 being more engaging. If I had to guess, I'd say Sennheiser made the HD700 to have most of the sound signature of the HD800 while adding a little warmth and extra thump in the lower end to be more broadly appealing. FWIW I did not hear any unpleasant treble spikes (I listened to the DT990 Pro and found those to have a much less tolerable high-end).