Illusion,
With $1500 you could replace the whole system! If you have $130 for trial, go for 1 set of the Vifa's at Dave's link. I am sure you will be impressed with the sound quality. If that's just barely not enough bass, buy another set for the rear. Otherwise, you'll probably want to investigate a new subwoofer with a powerfull amp that could overcome the back seat problem.
