It sounds like you're dealing with some kind of ground loop? Even having everything plugged into your power conditioner with their electrical ground pins can cause you these types of ground loops. I've had horrendous problems like you are describing when plugging in a laptop with its switching power supply. I almost always lift the electrical ground on the switching power supply to the laptop and all problems are then solved. Well everybody wants you to have properly grounded equipment, equipment and close proximity all plugged into the same power source do not all have to be electrically grounded. Otherwise, you may never solve this problem. So your mixer should be grounded and plugged into your power strip. But your compressor and anything else like your computer also plugged into the same power strip/conditioner should not pose any electrical shock hazards. See don't need to have all of the electrical ground pins plug-in. Get yourself a couple of those 3 into 2 electrical power cord adapters and give that a try.
Please note, you do not want to have different pieces of equipment plugged into different electrical outlets not on the same circuit. So for recording purposes, it should not be a problem. Coupled with the PA system, this could pose a problem. Simply because the electrical load of power amplifiers generally need something more than a 15 amp power switch/conditioner. So if everything is in a rack box together, start lifting some of those electrical grounds of the equipment within the rack box. And that should do the trick. Just make sure you don't get to loopy before you do these jobs LOL.
I was keep handful of the little suckers available
Mx. Remy Ann David