I have a couple Jensen subs very similiar to the monoprice sub......they r pretty decent for what they are, but I would browse thru the Partsexpress flyer and build your own sub. They are a fantastic company and you can pick up what you want for a decent price, especially if you have a little patience and catch their stuff on sale (sign up for their email flyer and deals).
Also look into bass shakers to install in the couch. Sounds ghetto, but I installed two (one really is plenty) in my couch and powered them with seperate APA 150 amps. Really adds another dimension to your movies. I got the amps and bass shakers from PartsExpress.
But if you are in a hurry, for the price, the monoprice sub is ok. It will definately help and you can always use it for another system or sell it if/when you decide to upgrade.
Have fun!!