Yes, in the middle of a square room is a problem. I've never had much luck with the sub
positioning you're using. I would try both in the front, or both in the rear to start with. Another possibility is both together in the front or rear, stacked or side by side. Sometimes one on each side wall works well. Your subs are different as to size and output so that makes it a little harder. There's also the crawl method, place the HGS at listening position, play some good bass and crawl (or walk) around the room and mark the spot (or two spots) where the bass is best and place the subs at that location or locations. In your case sub location is everything, so experiment. If this isn't clear, call me direct if possible.
Curt (928) 858-4430 Or email@example.com