I have my sub crossover disabled. The highest crossover setting on the speakers is 80Hz, which means that the sub is getting material from 80Hz on down (so an 80Hz low pass on the sub is unnecessary). The sub also gets the LFE track, which goes nowhere else and is intended for reproduction by a subwoofer.

If you add a second crossover at the sub, you have a couple potential problems. One is different crossovers at the same point interacting poorly (since crossovers are not brick walls but are actually slopes) - if the 990's crossover and a separate crossover are close (or especially if they are at the same point) you will have a dip in output. Another is that a crossover at the sub will cut out some of the LFE track - and that track isn't going to any other speaker, so that information is simply lost.