adding another sub?

I was wondering if i could add another sub to my system? I have studio 20's, servo-15 sub, cc470 center, klh rears for now. Was looking at a powered SVS pci+, would that sub go ok in my system? or should i go into another box sub? Love to booms in the movies.

...simple as that. Blending it in is another matter. That may take time.

I'm abit confused why all of of sudden so many want dual subs? I havent been in a room that was so big needed two when there are so many good ones to get.