Anyone Bi-amping with tube high & solid state lows

add an active sub to tubes and there you go...

Gain controll isn't a bottom line here.
The gain should be controlled dynamically unless the amps at the top and bottom end have absolutely identical characteristics.
To controll gain dynamically you need an active crossover and remove built-in speaker crossover.

a..a!
who sais that gain of amp is fixed?? the word "impedance" already dictates that the gain of amplifier is not fixed.
crossover interfaces as a "third-party" component limiting visible freequencies of each amplifier.
with no active crossover even slight difference in gains at the particular freequency can cause a great deal of distortions and loss of a tonal balance; realy, the tolerance is too damn small to even match to same models(must be matched to each other good good good).
and yes the gain is controlled by impedance and and impedance is depended on freequency that is controlled by active crossover.