OK so I moved the sub (only have one) to the half way point, and measured with the mic at the seating position (not 6' back). I did not adjust the sub distance in the pre/pro or change any crossover settings.
First graph is sub only and comparison between two positions
Second graph is Sub and speakers comparison between two positions
Combined with the speakers, the null seems to be gone. If you're willing to experiment, try sliding the sub in 6-inch increments rearward and forward of the 1/2 way point. A couple measurements in each direction might help fine tune subwoofer placement.