You don't need one of those amps for the HE400 though... But if I had £500 spare I would be interested in trying the TH600... Depends how much you are willing to spend.... TH600 probably has better treble, not sure about the rest of it as I have not heard them, although the D2000 which are Fostex headphones were very good and the TH600 are compared to the D7000.
Wild thoughts running through my mind. There's a shop not too far from where I live that sells the HE-400/500. I'll ask him for an auditioning meet before dropping coin on anything. Hopefully he can get his hand on a TH600 as well. Xmas is around the corner. But wait! Then there's the LCD2 hahaaaa