What is the best setting for an amp?

I'd think that first off, a comfortable listening level might be your best choice and where to adjust that depends on your speakers and how much power your amplifier has. Definitely not full volume as it can be rough on the amp and the speakers. Set the bass to what's pleasing to your ear, as well as the treble, etc. If you use a graphic equalizer, all the better for more accurate and pleasurable' adjustments. If it sounds distorted, it's not good and very fatiguing to listen to because you're either over driving the amp or exceeding the capability of your speakers, or both. Its' the same way with it being too loud too, as this will cause listener fatigue just as quickly as distorted sound. And if blood starts dripping from your ears, shut it off and get to the nearest hospital!