You may have read me already...
I've now received Nelson Pass new crossover kit and built only the input buffers so as to benefit from a 4 way JFET attenuator, then separated the filters sections of the M3TS so that I could use and match both Vidar and Aegir connected to the lowpass and highpass filter section (respectively) of each M3's
It's been 4 months of bliss now, the result is miles above my previous iteration (electronic crossover hackjob on a breadboard) or monoamping and I don't feel like the new Bifrost (since March) added much in comparison.
So... to try and answer you, I feel a good synergy exists when biamping the M3TS with both Schiit power amps.
Best of both 'previous' worlds really : better bass than when using the highs exquisite Aegir alone (sooo much better (even deeper and more articulate than with Vidar monoamping)) and better bass/mids/highs than when using the powerful Vidar alone, really ridiculously so... as in don't waste money on a Vidar if you have (really) efficient speakers and get an Aegir instead
I expect Aegir to excel on the X3 / X5, would love to hear it (or read about it at least).
Biamping the M3 Sapphires with Vidar/Aegir should also produce excellent results (caveat: Sapphire's Peerless tweeter appears to be 90dB/W...)