Putting subs at the mid-points of the walls will even out the response throughout the entire seating area. And the best way to tell is to use a SPL meter, at the seating positions, while running pink noise through the subs, at check the max output level.
You can also put a sub at the main seat location and crawl around the room, while sending pink noise to the sub and wherever the SPL meter reads the highest levels is where you want to put the subs. But if those locations are the corners, beware that corners can produce a too boomy output.