Live sound: How to set the levels on the power amplifier
The level control on your live sound power amplifier is, in most cases, a purely passive device. It doesn't control the gain of the amplifier - it controls the level of the incoming signal before it goes to the amplifier circuit. All it can do therefore is make the input signal smaller.
Let's assume for simplicity that you have one two-channel power amplifier for front-of-house sound, and full range loudspeakers.
Rather than wondering where on the dial the level controls should be set, the straightforward solution is to set them all the way to full. (Pull the master faders of the mixing console all the way down before you do that.)
The level in the room can now be controlled entirely from the master faders of the mixing console.
It may be however that a good level is achieved when the master faders are in an inconveniently low position, and small changes in position result in large changes in level. This would indicate that the amplifier is actually rather too powerful for your needs. There's no harm in that though - it just needs more careful control.
If this is so, then the level controls of the power amplifier can be lowered, both by the same amount, so that the master faders of the console can be more conveniently higher up in their tracks.
Where you don't want to be is to have the level controls on the amplifier halfway and the master faders halfway. That makes no sense at all.
In a fixed system, it is useful to fit locking knobs to the power amplifier so that they can always be kept in the same positions. Making a mark at the optimum setting is a reasonably good alternative. Power amplifiers should ideally be physically protected so only authorized users can change their settings.
Of course, individual models of power amplifier may differ, so it is always advisable to consult the manual for the manufacturer's recommendations.