Source?
Th600 drivers are about 105 per piece and th900 are 110 per piece. Much less to dealers. So while th600 drivers arent much cheaper would make more sense to use I doubt massdrop could obtain a large amount of th900 drivers due to the tougher tolerances on th900 drivers (heavily limiting stocks).
One poster here further back said thx00 had driver tolerances of 1-1.35 teslas. It would be impossible for the thx00 to utilize th900 drivers since they would need to be between 1.45-1.50 tesla to be even considered th900 drivers.
So design wise, the cups maybe similar to the Japanese cherry birch the th900 uses but I believe there is more differences that attributes to the sound of thx00 and thats includes the pads and potentially cup chamber depth.
Lastly, EMU Teaks measure cleaner than thx00s, especially with distortion. Their sound sig is similar as well. I assume this is because of the teak cups and cup dimensions. They must be using th600 drivers or lesser, highly leaning with something wuth less tolerances. Look at the th600 and th900. They both have similar sound sig, but the distortion on the th600 look like the distortion in the thx00 but the thx00 has much worse bass distortion. They maybe b-stock th600 drivers considering the price or thenlow end distortion can be caused by the pad and plastic insert inside the pads and the cups (it can be all three since the teaks have this too). One can even further speculate that thx00 and teak drivers are thebolder drivers found in the denons because they too all have bass distortion similar to the thx00 and teak this is worth considering even though all we can do is speculate
http://www.innerfidelity.com/images/FostexTHX00sn1927.pdf
http://www.innerfidelity.com/images/EMuTeak2016.pdf
http://www.innerfidelity.com/images/FostexTH600.pdf
http://www.innerfidelity.com/images/FostexTH900.pdf
