I am thinking about putting either an bazooka tube in between the bottom of the 2 rear seats. I will probably get an amplififed one. I am thinking about this becuase I believe that there is enough room for it under the seats. Also it won't take any on my cargo room up. Finally the baziika is aready tuned so you are going to get nice bass response out of it.
Rockford Fosgate makes a 6.5" sub, that should install right where the factory one is. Then remove the factory amp and put a bigger one in. I'll be using a Rockford 250.2. Hopefully this will give sufficient bass in my E. Hopefully have it done by april with my DVD system. Will update to let everyone know if this is a good idea.