Sony do a real nice 8" active sub that don't come in that big a box in the xplode range have a look as it would be a piece of piss to remove it and if you are giving up the car just crop off the cables and stuff them under a piece of trim they won't be found.

Other options are i have had a good pair of 6 by 9s that had a real good bass not close to a sub but decent enough.

Don't foget it ain't over till the fat bass sings.