The problem is the impedance of your speakers. Stock speakers usually are 8ohms or more. This allows cheaper factory head units to use a two channel amp and a speaker level fader to split power between two speakers (or more) on the same channel (front and rear). The new head unit has four dedicated amplifier channels meant to run at 4ohms (just like aftermarket speakers). What this does to the head unit'* amplifiers is it reduces the output by 1/3 (maybe more). You are over driving the head unit, trying to drive higher impedance speakers to achieve the volume that is expected out of them. The only way to rectify your problem is to purchase aftermarket speakers that are 4ohm.
Last edited by incognegro; 12-19-2009 at 01:45 PM.