Your HU will make quality sound up to a point, then it strains to push the volume and distortion sets in. Think of it like Pee Wee Herman lifting a weight - 5 lb is easy and you can smoothly lift it, and rest it just over an egg without breaking the egg. 50 is heavy and your arm will shake holding it, and the egg will be crushed because you can't hold the weight steady.

An amp is like steroids - now you're Arnold Schwarzenegger and that 50lb is easy to lift and balance over the egg, but 500lb is hard. So with an amp, your system can handle the volume because it has lots of strength. 5 lb is easier now, so the sound quality is better not just at 5lb, but all the way to 50lb.

An amp makes your music better because "regular" volumes increase from the 5lb point to the 50 lb point - wider range.

 

thank you very much guys