Every car I've every owned I've put a complete sound system in, deck, speakers, equalizer, crossover, amps, subwoofers. Except for the VX. When I bought it, I was pretty impessed with the sound; of course, I wasn't completely satisifed. Since the sound was pretty good and I leased the VX, I decided to only do a partial install. I disconnected the rear speakers (which generally are bad for the sound stage) and tapped into those speaker wires to feed audio components I installed in the rear. This consisted of an eq/crossover to smooth out the sound and protect the front speakers from bass, an amp for the front speakers, an amp for a sub, and a 10" sub. However, all this gear is working with the stock radio and speakers.

Of course, I had this gear lying around my house when I leased the VX. If you had to buy new gear, a 4 channel amp with built in crossover and a sub could achieve similar results to my system and cost a lot less.