How would the sub's crossover function? There are signal path issues that I'm not sure are solvable. If you set a 60hz crossover for the sub, what signal is that crossover applied to? Is it the sum of all five or seven full-range signals? If so, then you would have to set it to equal the highest crossover in the system or you'd lose information. Here's an example: mains set to 40hz, sub set to 60hz, surrounds set to 80hz - the result is that surround data between 60hz and 80hz is lost completely.

Here's an option that the 990 can do that might help you: set your mains to large, set the sub to L/R+Sub, and set the mains crossover at 60hz. This will give the mains a full range signal (which will naturally roll off around 40hz anyway), but a copy of the mains' data below 60Hz will go to the sub at the same time. There will be double bass, just as would be the case with the separate sub crossover, but data from other channels won't be lost.