Forgive me if you have heard this comment from me before.
Biamping, using a low level crossover, made a lot of sense back in the days when 15 watt power amps were common, 25 watts considered overkill, and IM distortion was one percent or more in the very best equipment.
Today, except for ProSound rigs where thousands of watts are required, the only real advantage of biamping is the elimination of passive crossovers. In the case of subwoofer crossovers, 100 Hz or lower, an active low level crossover and biamping is almost manditory, because the required values of inductors and capacitors for a passive crossover would be very expensive. The case for biamping at higher crossover frequencies is not so strong, although some (like Sean) swear by it.
Another factor, for subwoofers, is that a specialized LF amp can be used. All the audiophile characteristics like "transparency", "air" etc apply to HF amps. Quite inexpensive "plate" SW amps work well.