Using an aftermarket head unit with dual RCA outs and external power amps is the best set up. Remember, RCA outs are just unamplified signal, split from the source in the head unit before it hits the output ic. The reason you need high-low level converters from a factory head unit is because that signal comes out already amplified. So you need a circuit of resistors to drop the level into your power amp. The amp ic in your head unit is only about 5 watts RMS, whereas an external power amp can be anything!