Is the couch also near the wall? Room modes are far more common in the center of the room and in the midpoint of the walls, so I would maybe try moving forward and backward around the MLP to see if it improves.
Any chance one sub could go in the opposite corner on the opposite wall? (not likely I am guessing) And, room dimensions? Is the room more rectangular or squarish? When I had a nearly square room, it was a nightmare because my MLP was closer to the center of the room. Bass would be strong everywhere except the MLP, so I had to move my sub and my couch to mitigate it.