Two EP500s (though I've heard only a single EP500 and no EP800). I'd guess you'd get comparable volume and bass extension, but you'd be better able to tailor the sound, with two positions to adjust, two volume controls, and two sub crossovers. Also, if one breaks, you still have bass.

Greg
VP180, M80s, M22s, QS8(4), CSW S305s, EP500, Pioneer VSX-90
M2i, M3(2), Pio vsx-1020