I run a little 6" driver off the 100w (high) amp. and a 1x15 on the bottom(300w). Its my poor-man's fEarful 15/6.
But id wager it sounds just as good,,or pretty close, anyway.
I purchased a small, prefab cab (9"x9"x8") for a 6" driver from amazon, and got the 6" driver (used) of the bay for $20. No need for a crossover; its all done in the amp.
Theres really no point in investing alot of money for a bass cab to run off the 100w portion of your 800RB, imo. I mean, in biamp
mode, the high end is only reproducing mid to high frequencies.
Also, whatever cab you choose, keep in mind that you cannot run a 4 ohm load on the 100w high amp! GK says to use 8 ohm only