I went to the local audio store today to give the HD800's a go.
First impression when I saw and held them was that they're smaller than I expected, and felt and looked cheaper than pictures on the net conveyed. I think K701 and HD650 both look classier. Wearing the HD800, my ears basically touched nothing. There's a lot of room in those cups and in that regard, they've very comfortable to wear. They did, however, reach pretty low to my jaw and I didn't like the feeling of pressure there. I'm sure it's something one would get used to, though.
They were driven by a CD-player I didn't recognize, and a Musical Fidelity M1 headphone amplifier. I compared them back-to-back with my HD650's. I also have the K701's, but I don't like them very much so I'll not use them much for comparisons. The test CD's were mostly classical and vocal tracks, with some pop thrown in. The first thing I noticed about the sound, as have many others, is the amount of space. The sound was much further away than on the HD650's. It didn't sound like listening to speakers as the sound didn't come from the front, but it was definitely an "out-of-head" experience as far as headphones go. That's not necessarily a good thing for all kinds of music, though. They sounded quite Hi-Fi in the sense that they were explicitly clear and clean sounding, very detailed and well textured. The treble was infinitely more defined than on the HD650's. They were also very light-sounding. An uglier word for it would be "thin". There's a strong treble presence. Mids were smooth, but quite recessed compared to the HD650's. Bass was tight, but very lean.
In the end, one hour with albums I'm not familiar with, in an area that isn't completely quiet, isn't enough to form a final opinion, however the impression I got was that the HD650's tonality is more to my tastes. I would imagine the HD800's quite thin for rock, pop, and especially metal music. I can't imagine heavy guitars sounding very heavy or the drums very impactful on the HD800's. The HD800's were closer to K701's in tonality, however they weren't similarly plastic or nasal in the upper midrange, nor shrill in the lower treble. While the HD650 is very enjoyable, fun and involving, the HD800 is more like an analytical microscope. It's good for picking things apart, but I'm not entirely sure if it's all that much fun. I'd much like to have similar definition but in a completely different tonality. That said, I'm not counting them out yet. I hope to get another chance to listen with my own familiar music and perhaps EQ it out to match my tastes better.
HD800:
+ Clarity
+ Imaging
+ Comfort
+ Definition
HD650:
+ Impact
+ Presence
+ Fullness
+ Involving
My opinions, YMMV.
