The 460 comes with 4 tweeters ( 2 front 2 rear) and 4 woofers ( 2 front 2 rear). The doors have woofers in them with the tweeters up top, and the rear deck should have 4 speakers in it as well. The 4 woofers originally ran off 2 amps in the trunk and the 4 tweeters run off of the headunit. From the way it sounds, the previous owner either
A) ran the 4 speakers wires from the head unit to only the front doors
B) they somehow lost connection to the tweeters in the rear and one amp is running the 2 door speakers.