If you have a null that remains whether the crossover is set above it or below it, then the null is presumably created by the room. Moving the subwoofer is a possibility at that point (Outlaw's guide to the SMS-1 talks about using the SMS-1 to refine subwoofer placement), assuming you have options for changing where the sub is located, but it will only help if the null will be below the crossover point. Otherwise you'll need to look at room treatment or see if the Onkyo 805's Audyssey can address the null.