My gut reaction is that your problem is not so much the capacity of the sub to produce sufficient volume in your room, but the accoustics of your room itself. You may have a substantial suck out in the low bass due to room placement, location of the listening position, or the room's dimensions.
Buy a Radio Shack sound level meter and a test disc with bass tones and shck out your set up and experiment with placements and see what happens before you venture out buy bigger subs. Your room really is not that large.
Buy a Radio Shack sound level meter and a test disc with bass tones and shck out your set up and experiment with placements and see what happens before you venture out buy bigger subs. Your room really is not that large.