I have the 8 or 10 speaker set up without the amp, at least its not located next to the battery. After talking to an audio friend, he recommended I add a 10" sub in the trunk, since I am only looking to add a little more sub.
I am thinking one of these:
10" sub
http://www.jlaudio.com/acp110lg-tw1-...-systems-93332
8" sub
http://www.jlaudio.com/acp108lg-w3v3...-systems-93330