Mustang GT100 sudden volume increase

Have a Mustang GT100 3rd Gen type PR4400 that increases its volume after any note struck after warmup. Called Fender & they walked me through a software update which seemed to work for a while but now it’s back to doing it again. I’m capable of removing & replacing components if needed (I just don’t know what component is causing this). Can anyone help me with this or tell me what components values need to be tested? I have limited testing equipment (Fluke multi-meter, soldering station etc) but no scope. Any help would be greatly appreciated. Thanks, Ronny Dean