- If you have a 100 W amp applied to the bass and treble you get 100 W and that is the maximum seen by either frequency band.
- If you use two 100 W amps the bass sees 100 W and the treble sees 100 W. There can be headroom benefits with an active crossover but the max power each frequency band sees is still just 100 W. There is no more voltage to be had from the amps, you don't get 200 W to anything. Using passive bi-amping as implemented by the vast majority of AVRs there's not even a significant increase in headroom because both amplifiers still see the entire signal band.
- If you really want to play louder, get a bigger amp.
Even without removing the passive crossover inside the speaker, assuming there are terminals for the HPF and LPF (bass and treble) on the speaker, an active crossover before the PAs will improve their headroom (that of the power amplifiers). That does offer some benefit, though of course how audible depends on other things and is (as always) debatable. I have always seen (heard, measured) the biggest benefits after removing the passive crossovers and driving the drivers directly.
I hesitate to bring it up, but there are potential theoretical
benefits from passive bi-amping due to lower power demands. I cannot imagine audible benefits, however, for any reasonable system.
All IMO since I've learned everything is an opinion including all the physics and engineering I've ever learned...