Your subs have no idea where your couch resides, so the subs need to fill the *entire* space, 13X36. That's a large room and I suggest you buy at least two of the largest subs you can afford and if you care about how good they sound I would budget in some measuring equipment, such as Omnimic http://www.parts-express.com/pe/show...number=390-790 , to optimize their postions in the room.