Pete,
I had an hour on the HD800s yesterday. I took my HD650s along, as I could connect both to my Lehmann amp and that way could measure them against each other. I've read a lot of websites about the 800s, and as well as a lot of hyperbole, there also seem to be a lot of people having a dig at them, so I was definitely open minded before the audition. I took my own CDs along, and I listen to Kraftwerk, early Human League, Lou Reed, Grandaddy, Joy Division, Leftfield, Nick Cave and a bit of Kate Bush (good for vocals).
Straight away is the fit, they are unbelievably comfortable. The soundstage is awesome. Amazingly, my 650s sounded 'muffled' and constrained by comparison. However, I was expecting this, due to the fact that they retail at £1k. They dig out an amazing amount of detail. I would get something on the 800s, then switch to the 650s to find out if they were also digging out the detail. On most occasions the 650s did dig out the detail, however I had to listen for it, but with the 800s it was not a matter of listening for it, the detail was just there. I have read that they are not especially good at bass (or that other headphones do it better), but I thought the bass was excellent. When I switched to the 650s, it felt like they were trying too hard (if you get what I mean) and the bass felt woolly. This is from a pair of headphones that I love. I was told that the earpieces (probably not the right word) on the 800s are angled so that you don't get that headphone sound that everything is inside your head. Well, I didn't get that at all, and at no point did I feel the music was in front of me, like you get with speakers.
One thing that must be taken into account is the amount of leakage. I wear my 650s in the front room when the wife is watching tv, and although they leak, she is able to turn the tv up, and I get away with it. The 800s leak like a government minister! If the volume was up to any respectable standard, then anyone else in the vicinity will get a very good earful of what you are listening to.
Are they really more than 3 times as good as the 650s (as a price comparison, Sennheisser obviously think so)? I would say no, but they are beautiful, and the amount of detail they dig up, and the soundstage are beyond anything I've heard before. All things are relative, and if you are willing to pay £1k for them, then they are worth it. I'm considering buying a pair, but need to butter the wife up! I will also be selling the 650s as I'm not going to start collecting open backed headphones.......the dosh will also go a little way to paying for the 800s.