An external amplifer can work wonders. Most head units will do 12-18 watts of continuous power (even though they claim 50 watts x 4). However, that is usually at 1% thd, or where the amplifier begins to clip, and usually with 1 channel driven. An external amplifier, even one that does a measly 30 watts per channel rms, will make the system sound much better. It will improve bass response, dynamics, imaging and more. Installation is just as important as the product itself. Using some sound dampening material on the doors or rear deck can help out a lot. If, you are not comfortable installing, find a good, reputable, install shop to help you out.