Passive bi-amping is analogous to mixing PC2100 and PC3200 RAM in the same computer;
the PC2100 will limit the PC3200 to PC2100 speed.
Your smallest amplifier will clip before the larger amplifier, effectively converting it to a smaller amplifier, since both are running the same full bandwidth signals.
There are no real advantages I can think of to passive bi-amping.
It certainly does not increase the dynamic range.
Gain is the only real issue; a mismatch will create problems at the crossover frequency.
Active bi-amplification is different, since each amplifier is limited to its own frequency range by an electronic crossover. In this case the high frequency amplifier can be smaller since the power demands are smaller.
Active systems use amplifier power more efficiently, since there are no passive crossover networks in the signal path dissipating power.
They also provide much wider dynamic range, since the bass amp can clip all day long without overheating the tweeters.
BTW, amplifier power ratings are exactly that; they are just ratings into an 8 ohm resistor.
The actual output of an amplifier will vary greatly depending on the load and amount of current it can provide.
ie: two identical amplifiers will have very similar output; two amplifiers of different manufacture that carry the same output rating will not typically have the same actual output.
The difference between a 100 watt amp and a 140 watt amp is exactly the same as the difference between a 10 watt amp and a 14 watt amp. (all other factors being equal)
ie: the difference is negligible at best, and probably not audible at all.