hmmmm.. you mean the frequencies below your surround and center speakers crossover are sent to the mains AND the sub if your mains are set to large?
I am not so sure if the surrounds and center would be directed to the mains, the sub for sure, as any speakers set to small have anything below the XO sent to the sub, as we all know, whether or not the surrounds and center are mixed into the mains channel as well......must experiment.