It's easy enough to bi-amp. The hard part is finding the two cabinets that are complimentary to each other to work as a unified piece.
In the late 70's, these rigs began to sprout as their were no powerful dedicated bass rigs, save for a few old warhorses like SVT's, Sunn or Peavey Coliseums or Festivals, or the Acoustics. Usually hooked up to folded-horn 18's !
In sound reinforcement, there are lots of cabs built specifically for their intended range as parts of systems of cabinets designed to work together (think Meyer, EAW, JBL, TurboSound, etc.). In bass guitar cabinets, there just isn't a lot of system-think for cats that want to bi- or even tri-amp a bass guitar rig. Some of the builders do sell subwoofer-ish cabs to add to their full range bins, but it's an add-on situation, not optimal. Then, past whatever the specs say, does it sound musical? Even with my unsophisticated ears, I can hear the time-smear of the subwoofer dragging in after the hi-pass fires on some rigs. And as others have pointed out above, the crossover is all-important to tone as well. But they're third party gear. As you can see, trying to blend all these things can take quite a search.
I've had them before and have never felt led back to them.
However . . . PA bins are going to power amps built in. So I am very interested to try a 2-way rig daydreamed by me, and confirmed by a recent converstaion with the wizards at Bag End:
Bag End now builds their ELF subwoofer with the ELF circuit and a power amp and a crossover built in. Couple this to their 210 with HiDrive and the same power amp built in as well . . . systemized, tuned for those particular cabinets, this is sounding REAL interesting. Not to mention the inputs can go line-level where I don't even need a preamp, if I want to go 'straight, no chaser' like our friend Jimmy J.
Will HAVE to make the next BagEnd get-together!
(Note: I know BagEnd no longer call it ELF, but I do!)
J o e y