most any properly installed amp will sound better than the headunit for a number of reasons:

- better power supply
- headunit manufacturers tend to stretch the numbers on the rated output. You are almost certainly going to get more power from a separate amp.
- typically better noise separation
- Since the amplifier doesnt have a CD motor servo and an FM signal amplifier in close proximity to it, there is less noise introduced into it. Of course, if you use crappy RCAs it defeats the purpose.