to add an AMp to a factory head unit and accually have it sound better, there are two methods..
The old school method (which would be best IMHO) is to also buy a 3 way cross over, wire you front and rear spreaker wires from the back of the reciever to the crossover in proper phase, this will give you the ability to convert your signal into # different levels of Htz 100 and up Your "Highs", 30-90 "Mids" and 0-30 "Lows" This also will convertthe signal to a RCA style out put so that it can be wired into the Amp with a lot less RF interference.
Down side is that this method is a little pricey.
The second would be to use Low Pass Converters, they are cheap and effective, but do not seperate the different levels of sound, that are also very linier sounding and do not allow your to fine tune your music. they will also allow some RF interference. these are the type of converter that gets tapped into the speaker wires behind the radio and have two RCA outputs on the other side.
Most amps do not have line level inputs anymore (just attaching speaker wire) the older amps that did have these were not that great cause of the amount of RF interference that they allowed.