[mid_life_crisis]

This is going to be old news to some, new information to others, but there is a proper sequence for adjusting gains for cleanest sound. You first need to establish at what volume the head unit itself starts to distort. You do this by leaving the amp gain all the way down and listening to the sound on a high quality recording and slowly turning the head unit up until the sound starts to go bad. Turn the head unit down until it goes away. Make note of the volume. Never turn it up higher than this. Now slowly bring up the gain on the amp until the speakers it is driving blend properly with any other speakers in the car. With luck this will happen before the sound starts to distort. If the sound distorts first, you need a different amp because whatever you bought is over matched by whatever is driving the rest of the speakers.

Edit: This should be obvious but just in case.
Distortion may not be the H/U or the amp. It could also be caused by trying to make cheap speakers play too loud or too low for their design.