If you replace factory speakers and amplify them, you desperately need either a new head unit (not an option in your case, it sounds like) or a sound processor. I installed a JL XD700/5 and JBL MS-8 processor and it was a phenomenal improvement. I, too, wanted to retain the factory head unit. The JBL lets me listen to the system while bypassing the processor just to hear what the replacement speakers and amp sound like (and it's a good amp), and the sound is pretty terrible. The processor is needed in order to restore the signal downstream of the factory amp, which cannot be separated from the factory head unit. Hope this is helpful!