If you are not too worried about losing the parcel shelf storage space it is possible to fit a subwoofer to the car!
When I purchased my Cappo about four years ago it came complete with a sub box that had been built around the centre section of the parcel shelf. As it had been custom built for the Cappo the previous owner had left the box in the car complete with an 8" speaker. It seemed a shame to see all his hard work go to waste so I decided to build up a new system around the already fitted sub.
The box is covered in the same material as the floor and blends in nicely with the rest of the car and the sub points out forward between the two seats. A pair of 6 1/2" mids sit in the the standard slots in the footwells along with a pair of tweeters that nestle in the slots built into the top of the dash. The whole system is powered by a 4 channel amp that sits behind the drivers seat with 2 channels powering the front speakers and the other 2 channels bridged together for the sub.
By the way, I have looked into getting a spare parcel shelf for the Cappo so that I could whip out the sub box when I needed some extra storage space. If my memory serves me correctly the centre section of the parcel shelf costs around £60.00.
Mark