I bought this:
2 x 12" JBL Subwoofers in enclosure. GT5-2402BR- $200
If you seriously want some bass.
http://uk.jbl.com/jbl-produkter-deta...t5-2402br.html
And I havent used it.
It is big and in reasonable condition - there are some small marks on the box which is how I bought it. It is huge too so you will lose your boot but what a way to go hey!
Can take it out if you want to eat or use your back seats...
PM Me!!!