Personally, that is not my experience. To me anything from the Rx Mk3 to a Zana Deux the TH-900 doesn't change that much. But I'm the kind who is willing to spend a lot more in the amp just to squeeze the last few percentages out of the TH-900.
The TH-600 on the other hand to my experience benefited from different amp electronics. With Fostex own HP-A8, I felt the TH-600 matched its price with respect to the TH-900 pricing. On the other hand when I changed to a different DAC/Amp the TH-600 sounded much closer to the TH-900, that made the TH-600 better value than it's priced.
In other words to me the TH-600 is able to scale more than the TH-900, and the TH-900 doesn't need expensive amps to achieve it's capabilities.
I have no experience with the th-900 but I do have quite a bit with the th-600 and have found it to be a pretty good scaler myself.