Dave and I have been exchanging a few ideas offline and share some similar taste in equipment. I think biampable NXOtica’s are a great foundation. At 93dB they can be driven with low power tubes but not necessarily flea power tubes.
My recommendation is to use the tube integrated or pre/power amp of your choice (300B in this case) to power the MTM section, and use the speaker-level signal to drive the input
of a Firstwatt F4 (or F4 clone or a Burning Amp Clone with output section only). The F4 is a zero-gain design, so it will match the tube amp’s output perfectly and pick up its sonic character while being better suited to drive the our woofers. There are a number of builders who would put together an F4 (parts are <$1k) and custom-build a SET amp as well.
The F4 manual includes connection diagrams for driving the amp with the output of a flea power tube amp: http://www.firstwatt.com/pdf/prod_f4_man.pdf