So, the idea of matching amp output impedance with speaker impedance is possibly one reason why going to an active crossover works so well with tube amps? For example, I recently bought a DEQX, which does preamp functions, crossover functions (I set my at 96db/octave slopes), phase correction, group delay correction, time-alignment corrections, and frequency response corrections (both inside of each drivers passband, as well as between the different drivers). I have a 4 ohm tweeter, which I use the 4ohm output tap from my dedicated tweeter tube amp, and I have an 8ohm midrange driver, which I use the 8ohm output tap from my dedicated midrange tube amps.
Is this essentially the approach you are advocating here? If so, it would explain a lot of the dramatic jump in sound quality when I went active and was able to do phase and frequency response corrections.