I have been looking all over for this and have found everything except what I want to know. From what it looks like, the factory installed sub is about 6.5".

What I want to know is if i can get an aftermarket sub to fit the factory opening and install an aftermarket amp as they did with the factory bose system. If so, are the rear speakers able to be hooked up to the new amp, or will the new amp only be pushing the new sub. I just want a little more solid base as the highs seem to be fine (for now). Although in the future, I plan on adding new components all around.

OR, could I simply install an aftermarket amp to push the factory sub a little harder? Any help would be appreciated!