My brother sent this unit to me to have repaired, but I wanted to see if it might be something simple before taking it to my tech. Looks like this model doesn't have adjustments for the amp section though. My brother says after playing for over 15 minutes, it sounds like someone turns the volume down 50%. He has fiddled with the switches to see if they were causing it, but he says it has no effect. Seems to be heat related, and in both channels. I just wanted to see if anyone might have an idea what would cause this.