I can't get enough of them. I've just passed the 200 hour mark with the Senn's, 90% of that with my ODAC/O2 and they sound wonderful! I hear people say the 650's pair very well with certain valve amps but I'll be damned before I feel as though anything's missing right now.
A lot of people who like valve/tube amps will say the same for most headphones not just the HD650. I for one don't want extra harmonics or "pleasant distortion" added to the original music.