The standard approach for setting up one or more subs in a home theater system is to let the surround processor do the bass management - in the case of the Model 990, that means setting the fronts to "small" and selecting an appropriate crossover point. For speakers rated down to 80Hz, that probably means a 100Hz crossover point or possibly even 120Hz. The subwoofer's built-in crossover could then be disabled, since the processor is taking care of controlling what frequencies go where. If you have two subs, they would typically share the same signal and thus handle the same frequencies, unless you went with something like HSU Research's mid-bass module in conjunction with a traditional subwoofer. (In that case, you would have two different subs each handling a different piece - the true sub for the really deep stuff and the mid-bass module for frequencies closer to the speakers' bottom end.)