getrippedoff, before anyone can seriously recommend you a subwoofer to look at we need to know what it is you primarily listen too as different speakers suit different music.
Marty, as for Alpines having the smallest enclosures of the common market; they do have small enclosure, but not the smallest. The Hertz high energy series 10" can run down to 0.2cf (obviously the magnet has to hang out)...
getrippedoff, you can fit a 10" under the shelf without dramas. This is the last one we did:
The amp if small enough can go next to the spare tire. Again, this is the same car: