I made some measurements this morning, making sure my proposed placement in the ceiling is still the best spot according to our remodeling plan. The best spot won't work of course, rarely does in a non-dedicated room. The room modes are going to be the issue with these subs, but I think I can make it work really well with some EQ from REW to the miniDSP. I placed the horn mouth at my proposed LP and moved the mic to the future placement of the horns.
Green - left upper corner
Red - 1/4 space to the left on the side wall
Purple - current LP with the sub on the side wall to the left
Blue - 1m ground measurement of Gjallarhorn in the middle of the room
Modal calculations by REW room sim