I use two F113's. The point is not to shake the walls but to even out the frequency response in the room (always easier with two subs). I would buy one F113 now and when funds become available buy the second one later. You can run them in stereo (master/master) or as master/slave (which I do).
By the way, if the walls are shaking and you are not watching the pod scene in War of the Worlds you are running them way too hot and/or you have room mode issues to fix.
By the way, if the walls are shaking and you are not watching the pod scene in War of the Worlds you are running them way too hot and/or you have room mode issues to fix.