That could be an interesting amp to try. I bought a Pass Labs XA 30.8 for use with some high sensitivity speakers, and also I tried it out with the TAD's. It was a bit warmer and "gauzier" in the upper bass, low mids than I would like, and it seemed to lack dynamic spark. Now, it could just be underpowered for the TAD's, clearly, but it was not a good match. So far, the BHK mono's have been the best with them.
As with all things in audio, you do hear some alluring aspects from the M900u that make you want that, but mixed with the best virtues of the other amps you try - - keeps us wishing, doesn't it!