| the front speakers are wired to a crossover in the factory amp. So maybe you canceld the crossover when you installed the aftermarket amp. You had to rewire the conections from the factory amp to the aftermarket amp. and im not sure how to do that!!
What i did in my car was put a wavebox in the trunk with a 10" woofer and run the back speakers directly to the Aftemarket headunit, the aftermarket amp to the crossover in the factory amp. and then i canceld all the bass to all the speakers and once i took the bass out of the factory speakers they sound pretty god much more crystal sound and i have plenty of bass from the woofer.(i took the insulation out from the trunk to get more bass) | 
