I have had my TH600s for 2 weeks now. I have upgraded from Ultrasone 900 pros and I have them running through an April Music Eximus DP1. I have also tried them though a little dot III.
I had done the kees mod to the Ultrasones as without it I found the bass and treble too intense. I also replaced the headband padding with a Sennheiser 650 HD headband pad velcroed in place.
Like this I could and did listen to the Ultrasones all day. I must have a sensitive head, because with the Fostex I ended up with a bad headache after a couple of hours; so I have wrapped the trusty sennheiser headband pad in a Leather off-cut I have velcroed in place; and I have been listening all day. No headache.
As to the sound; well they are simply awesome. Compared to the Ultrasones (which I loved) the sound is just so much bigger; dynamic; fresher. The bass quantity is similar but just goes so way much deeper. They took about ten hours to settle down (or for me to adjust to their sound). The Ultrasones had a decent soundstage, but the Fostex is so much bigger with more defined placement, ironically I had a test track with the ultrasones dynamic experience come up on random play; suddenly it felt like I was sitting in the middle of an Oompah band, you could almost smell the lederhosen - eaargh.
As others have noted they are not the most isolated closed cans; certainly not as much as the Ultrasones; where the world was pretty much shut out.
I just downloaded Photek DJ Kicks from Bleep.com in flac - wow! Everything I have been listening to is wow. These phones just turn your senses to 11 and keep them there.
I must admit I was dithering for months about buying the TH600 or TH900. In the end I though get the TH600 give some of the saving to charity and spend the rest on Music. I think I made the right decision, 'cos if the TH900s are as big an improvement over the TH600s as the 600s are over the Ultrasones I think I would have exploded.