I found this SUPER little device that's handy-dandy for live shows. It's the SM Nano Patch + SM Pro Audio
We are a 3 piece 50-60's R&R and Classic Country group. We set our volume levels with a SPL meter and try very hard not to be TOO LOUD and make sure our vocals don't get buried in the mix. This little 2 channel device goes in-line between the mixer and the powered speakers. We set it mid-scale, then ballpark levels on the powered speakers. (We've set up enough times that we have "typical" levels marked on the mixer and speakers that make it pretty easy to get the levels fairly close the first time.) Once that's done, it's a simple turn of the knob to lower or raise the mains w/o changing anything in the mixer or in the monitors. It even has a mute switch which completely kills the mains at the touch of a button.
It's a great investment for the small group with no sound man.