Scott,

Outlaw recommended that I set the sub's crossover well above the 990's crossover points for the speakers. If I understand it, the 990 handles all crossover duty and setting the sub's crossover higher than the 990's speaker crossover settings gets the subs electronics out of the way.

Do you have access to the response curves for your speakers? If so, you want to set the crossover above the point where the speakers start to roll off. If not, you could try the mains at 80Hz, the back speakers at 60Hz, and the center at 80Hz. With higher crossovers you rely on the sub to produce what could be directional sounds. Give a number of crossovers a try and see what you think.

For the sub I'd use the test tones first and then do a lot of listening with music you know and set it accordingly. Follow up with movies that have a lot of bass/LFE. You'll eventually find the right setting. I'm sure other Gunslingers can offer a more scientific approach to setting the sub. Enjoy!