Thanks for the detailed impression, MF. I wonder how the TH600 and 900 stack up against the German flagships, like the T1 and HD800. My T1 pretty much blows my stock D7000 out of the water for balance, sound staging and accuracy.
It seems that the Fostex TH line has less recessed midrange than the Denon Dx000 line, however David in his flagship comparison thread said that the TH900's midrange is actually more recessed than that of the D7000. I wonder which one it is.
I think it depends on what you're looking for. If you're looking for a balanced headphone, the Fostex TH series won't stack up well just like the Denon D series doesn't because they weren't made to be balanced headphones.