I just installed a new 990 in my living room system and went through this with an Infinity ssw 210 powered sub. The sub has a variable cross over and a volume control built in.

My main speakers are Magnepan MG 1.6QR's which go down very nicely to about 35 or 40 hz so I just want the sub to kick in about at that spot. I set the subs crossover at 100 hz to basically by pass it, and the cross over in the 990 at 40 hz for both the sub and the main speakers.

Scott