The other thing we all need to keep in mind is price...
TH900 new is more than double the cost. Some people don't have to worry about that, but others do. You will obviously get a reduction in performance with a reduction in price, common sense.
You wouldn't buy a $25,000 car and expect it to drive like a $50,000 car. Nor can you buy a $400 headphone and expect it to be on the same level as a $1000 headphone.
Same with any brand - HD700 doesn't match the HD800, right? TH600 and TH900? Similar concept. This is something we all know but I think can get lost and forgotten in the comparisons when strictly comparing sound. You're talking about a much cheaper product.
More options is better for us consumers. You can start with something like the THx00 and if you're intrigued and want to go further, you can eventually move up if finances permit. You've also got people moving in the opposite direction. You've got a great accessible option for many in the THx00, that like others have said, will only further stimulate interest in the Fostex brand.