I've been receiving questions about the Fostex TH-X00 vs. the JVC HP-DX1000 (and TH-600) and thought I'd repost a PM in here:
The Fostex TH-X00s are still burning in. They have about 20 hours of burn-in right now. My TH600 took >100 to settle down completely, although the changes were still relatively minor.
For me, I still prefer the way the DX1000 conveys acoustic instruments and strings. It has more stock bass, which is great for my tastes, and a bit of a cavernous effect with its larger wood cups and wood-mounted driver (which I like, but some people think is undesirable resonance).
That said, I think more people would prefer the TH-X00 overall to the HP-DX1000. I have specific tastes like emphasized bass and rolled off treble without dark mids that the DX1K does very well.
The TH-X00 is less bass-forward and more treble-forward comparatively, which is going to be to the tastes of most.
Interestingly, they sound almost the same as the TH-600 if you swap the pads out to HM5 or Audeze, both of which I used a lot with the TH-600. Basically, with some wood cups and TH-X00 pads, they might actually be the same headphone. That said, it would cost you ~$250 to convert the TH-600 to sound like these (assuming you can find the same pads, which you might not be able to).
If you want a complementary headphone to a TH-600, the DX1000 is more different, while the TH-X00 is about halfway between the two in sound signature.
Here's a super crude paint of roughly how I hear the three headphones in frequency response: