As an ex audio engineer who was often telling people like you who would never listen to keep on-stage volumes down, I can tell you that after a few tries I'd give up and just let them have their crappy sound. There's so much talk about "amp in the room" but not nearly enough talk about how bad most bands sound on stage as a general rule. Guitarists aren't the only ones to blame, but I'd say that more often than not, they're the prime problem. Yeah, it might sound great to you as you're next to that cranked amp, but the end result is a poor quality mix for an audience. If it's feedback you're after, buy a Sustianiac. I swear by them.