I was looking for a showlow mount sub as well (Blaupunt & Pioneer both have them), but decided to go another route.
I ordered a Pioneer GM6100F amp to replace the stocker (following shopdog's lead on that), will replace the door speakers with Kappa 6 X 9's 2 ways down low and 3-1/2's in the upper spot, 5-1/4's in the rear (rewired for stereo), disconect the center speaker and add an Infinity BassLink into the rear compartment which is a self powered sub (and it can be removed if I need the room).
Not sure how it will sound, but it'll be better than stock and I'm told it should actually be very good?