I am strongly considering a second 15h (trying to convince my girlfriend to get me one for christmas) for my theater. dimensions are 10 3/4' width, 16' long and 6 1/2' tall. I have a 7.1 setup and its awesome now but still want more bass. Girlfriend says its enough, i strongly disagree

and want a second. What type of output am i going to be looking at with an additional sub? I know its relative but any ideas are helpful. If they arent going to be large i am going to invest the $1000 into new speakers or amps or something.