[lincpimp]

You really need an efficient sub to go along with that am. Something with over 90db sensitivity rating.

And you can certainly leave the stock rear speaks and just upgrade the front. Reas are just for fill, and do need to be crossed over pretty high. You can also trim off the top end frequency of the rears (10k and up is a good start) so you just have a midrange sound coming from behind you.