I have seen many bass guitar cabs with different drivers, perhaps a 15" or 18" with a pair of 8" or 10" for midrange punch. I don't know if these are crossed over or simply wired parallel.

Due to changing role of bass guitar in pop/rock, there is a trend away from the heavy boomy bass, to a tone with a more crisp and precise sound... thus multiple smaller drivers. You see bass cabs now with four 10's, for example.

Paul