The biggest advantage of proper gain structure is dynamic range. I really doubt you are concerned with that.
There is nothing you can do-or nay setting you can make that will allow you to get more level out of your system-that is determined purely by the cabinets used and the amplifier power.
But you can make the quiet moment quieter-by lowering the noise floor. I get the impression there are not many quiet moments during your events.
Why bother measuing the input voltage-it is what it is and you can't do anything about it.
And if it is a little bit low-I really doubt you will be able to tell if you are not getting full power to the loudspeakers. It would only be a couple of dB at most.
Don't waste your time on things you cannot do anything about.
You can't "make sure" you are getting the proper voltage.
What are you going to do-call up the power company and say "Hey I'm Jack and I am at so and so address and the voltage is a tad low-can you crank up the voltage at the plant so I can make my sound system as loud as it can possibly go"
What you need to do is to bring more rig than you need. That way you don't have to woory about the voltage so much (It sounds like you are going to use it as an excuse "well it wouldn't get as loud as I want because the voltage is low"
NOT!, AND you don't have to worry so much about tearing up your gear-whcih I get the feeling is going to be a common occurance. It will be if you always run your rig at full potential.