Mat, might be worth a look at an under seat sub setup? It's far more subtle than a big old wedge in the boot.

This sort of thing http://www.caraudiocentre.co.uk/cate...ers_c-2741.htm

Like the look of the kenwood on there. They've got built in amps and that one has a little remote so you can quickly adjust the gain. Not gonna win any comps with it but I love the idea of these to just add a bit of tidy bass to a system.