Typically biamping can lead to sonic gains if all the pieces are of at least good quality.
I am a little confused as to what your connection diagram would look like. If I understand you correctly, you would have:
- source=>preamp=>amp=>external xover=>speaker driver? This would still connect to the drivers binding posts, and go through the internal crossover. External crossovers can be used to feed the drivers directly, but would probably require speaker surgery as I don't think the Axioms will allow direct driver feed.
If however you mean this schema:
-source=>preamp=>external xover=>amp=>speaker driver, you still hit the internal crossover. The amp is working on the frequencies you set at the upstream crossover, which can make an amp under used. This can also void warranties, and sometimes the designer has used the internal crossover to align phase
Sonically, the best method is to connect the amp to the drivers, and use the internal crossover.