The generally accepted practice with a PA system is to turn the amplifiers up full and do all the controling at FOH which can be just a mixer but can also include some external processing. This doesn't always produce the best signal-noise ratio but it's safer in that you don't have to worry about some mischevious fingers cranking up the amp gains while you're not looking which could potentially damage speakers.
This also relates to setting all the gain and level controls correctly for best system performance. If you search for articles describing "setting gain stages" you will find various theories on how to do it but in general the idea is to set the signal level at each stage through the mixer(and any outboard equipment) to hot but safely below clipping, and then bring the amp gains up to a level that produces the SPL you want or it's maximum clean output.. whatever comes first.
This also relates to setting all the gain and level controls correctly for best system performance. If you search for articles describing "setting gain stages" you will find various theories on how to do it but in general the idea is to set the signal level at each stage through the mixer(and any outboard equipment) to hot but safely below clipping, and then bring the amp gains up to a level that produces the SPL you want or it's maximum clean output.. whatever comes first.
this somewhat simple seeming question has a few caveats to it...some are tied to gain structure of the system as to what becomes appropriate for power amp input levels....another comes from loudspeaker capability with respect to bandwidth, as in: if the OP only has 12's does he expect low bass at high power? as a mid/top they would handle far more watts.