The existing door midrange and tweeters are good quality. They would benefit well with a good quality deck like the Kenwood.

There is just not enough space under the Crossfire seats for a decent subwoofer. The woofers behind the seats would sound better than any small speaker placed under the seats. The system in the car was designed for mass appeal, but sounds better than most in its class. If you have an ear for music, you'll know that the bass in the Crossfire just sounds anemic.

So, if you are looking for bass with impact and authority, it is best to install a dedicated subwoofer in a sealed enclosure, with the speaker having at least 8" in diameter and powered by an amplifier with at least 200 watts of power. This system, of course will not fit under your seat.