[ddouglass]

Even the "expert" can't agree, eh George. I guess it is that way in any technical situation. You go with what sounds best to you and your customers in our case.
Now as for the Amplifier settings there again it is up to you to find the optimum range for your amp.
First off from purely a technical view an electronic amplifier circuit be it solid state (transistor) or older tube version is at its peak efficiency with 75% power applied. Beyond that the amplifier circuit will clip and distort the signal.
However depending on the quality of the Amp unit (the whole box) you buy will determine your setting. The higher-end (quality not price) units are adjust so that 100% of you volume control is actually roughly 75% power to the amplifier circuit. This way you won't distort or clip your signal. The lesser quality Amps do not have this and will be much more likely to distort if you use maximum power.
George, unless the bands instruments are also fed through a sound board you won't see them run wide open because they do not have any other control on volume except the instrument amp. That said if you run you amp at the higher volume then control the amount of input with a sound board/mixer then you output will be a cleaner crisper sound at the volume level you desire. This also puts all your control (mics and music) in one location.