The biggest benefit of bi-amping is the filtering of the signal that is sent to each amplifier channel, and then to the speaker drivers. A 2-way outboard line-level crossover takes the full-range signal from the pre-amp, divides that signal in two, sending a low-pass filtered signal to the amp driving the woofers (the mids and highs removed from the signal), and, more importantly, a high-pass filtered signal to the amp driving the midrange and tweeter drivers (the bass removed from the signal).
Keeping bass frequencies out of the amp driving the m/t drivers greatly reduces the distortion produced by the amp, as well as providing much more available power for the m/t drivers, both of which results in cleaner mids and highs. Using a high-quality line-level x/o also provides cleaner sound by virtue of it eliminating the often-poor quality parts used in the construction of speaker-level crossovers, the greatest weakness of many speakers.
If done well, bi-amping will provide a greater improvement in sound than will mono-amping. But like I said, if you can afford to do both, so much the better!
