Well, when you go aftermarket, the amp is built into the deck. The Bose amp in the rear will no longer function.

Also, with your 6 speakers, I'm guessing you have 2 in the dash, 2 in the front doors, and 2 on a rear deck lid. The 2 in the dash are spliced into the front speaker's signal. When you play with your sound fader and throw all the music to the front speakers, you barely hear anything out of the 2 dashboard units. So, you do have 6 speakers, but 4 that really count.

With the speakers you have now, most mid level Pioneer and Alpine radios will surpass their capabilities. Depending on features you were looking for and price range, I could help you find something that fits your needs.