By active bi-amping, I was referring to eliminating the passive crossover network inside a speaker system (not the sub) and having the crossover duties performed by an outboard electronic one. This yields far superior sound than can be had by any speaker that has a conventional passive crossover network (almost all consumer speakers have a passive crossover network).

The benefit for the woofer is that the high damping factor of the solid state amplifier will control that driver very well, and the smooth character of the tube amp will be of benefit for the higher frequencies where the ear is particularaly sensitive.

Converting a speaker system for active bi-amping is not something you try at home however unless you are really devoted -

The other option is to use a tube amp for driving the unmodified (conventional crossover) front mains. As long as they are of relatively high efficiency (around 90db/watt), a good 50 watt or so tube amplifier will work great. Three monoblock tube amps for the L,C,and R speakers and a solid state amp for the surrounds is a good way to go. If you want to get really crazy, you could power the surrounds with tube amps too, but I think this is kind of overkill.

A tube amp is not a good choice for driving a subwoofer because of the high power requirements and the high damping factor needed to properly control the driver in a sub.