Hi Jerry
Looking at the manual for the stagepass it looks like you are hitting either the feedback suppressor or limiter on the system. Given you are perceiving the keys are being adjusted, and as the system does not have the ability to affect automatically a single channel, my guess the problem is the feedback suppressor kicking in. While my first gut instinct would be turn off the suppressor, given the nature of your service you may want to keep it functional.
Therefore what you need to do is readjust all your inputs into the system to ensure that they are not affected by what seems to be a normal operation. In your case this would be a combination of dropping keys/mic input levels and raising your master to compensate. As the connection to the stagepass speakers is through a DSP which applies a limiter after the master you may even feel free to turn up the master up to close to 100% (I would leave some wiggle room so maybe 90%) and lower all your input levels to compensate if that is simpler.
