Don't do it. I have all those, you are only getting a little better from the thx00 to the th900.
what you will get is a little more clarity maybe 15 to 20 % more, a lot more sound stage, sub bass. You will loose the mids of thx00, th900s is not as balance sounding as thx00.
thx00 is better value for sure. Unless you have the $$$$$$$$, go for it.
I got the TH600 and M-100 but i haven't been interested in the X00's cause of the 600's. The TH900's though, always on the radar. As for the difference from the 600's, I don't think I really care. If I can find them cheap enough this year, then they gonna get got....shout out to fostex, HBB, and Zeos.