The factory amps are designed to work with the factory speakers/subs and vice versa. Meaning, the speakers play at different ohm loads and the amps are crossed at different points. You might be able to throw some aftermarket speakers on them, but who's to say how long they'll last, not to mention they'll more than likely sound worse than factory. The only right way to do it is replace the entire system....deck, amps, and speakers.