Sub amps have a built in crossover - you can lower the bass in your headunit to stop the door cards vibrating without even touching them!
Matt, try making the bad noise in your car, then turn the bass on the set to middle of the road (or lower) and see how loud it can go before vibration / distortion stops you.
Normally this is quiet loud so you can use the sub to give you your missing bass.
I'd go for a self powered 6.5" or btter still the 8"Bazooka tube - they've been around for donkeys and have the amps / crossovers built in.
The sound from this tube will compliment the VW unit very well and they only cost around £150 incl postage from the states.
Install sound cost around £50-60!
IMHO
Good Luck
