Out of curiousity have you done the capacitor mod for the door speakers? They clean themselves up rather nicely after that mod...

edit: I would start with replacing the speakers alone if you do not like them. Make sure to look up the specs on them and whatnot before you shell out big bucks for speakers that may not work.

If you are still unhappy, move to pulling out the old amp and getting a new one.. perhaps to power just the sides or even to power the backs as well.

if you'd be spending that kind of money anyway, why not spend as little as possible (just the speakers) and then move up?

Though, if you haven't done it, the capacitor mod is all of $5 bucks and two beers. That'd be my first try. Followed by speakers, and then possibly by a new amp and old/new back speakers, etc.