Nice topic, i was recently changing my 4"inches on the back. and actually they are 3.5, which i dremel'd to make fit. But my sound actually still sux, for some reason i only her one side in the rear, and loudness is gone, i have to crank up to max to listen it decently loud . Could that be amp or its ohms of the speakers? I would like to know what is factory amps impedance, because for speakers is 4 ohms.