i know that Carl has nothing against bi-amping per se, but i believe it's better done the way you are suggesting (different power amps for different frequencies) with electronic crossovers and separate subs.
or alternatively do what was suggested earlier and use two of the same power amp, rather than mixing valve and solid state power amps, even if driven by a valve amp.
if you are set on using both valve and solid state power amps i suggest you try VAC's phi300 as it has a gain control, which will allow you to compensate for differing amounts of gain
but my instinct is you will be better off using matching power amps
or alternatively do what was suggested earlier and use two of the same power amp, rather than mixing valve and solid state power amps, even if driven by a valve amp.
if you are set on using both valve and solid state power amps i suggest you try VAC's phi300 as it has a gain control, which will allow you to compensate for differing amounts of gain
but my instinct is you will be better off using matching power amps