he knows best. If he asks you to turn your amp down, please turn your amp down, and be nice about it. Chances are that the sound guy knows what will sound better in the room more so than your local band that may or may not have played that bar/venue before.
Let’s take a second to discuss the dynamics of playing in a bar. The bar needs to make money. The band is there to provide entertainment for the bar patrons and make the bar money. The sound guy is responsible for keeping things at a comfortable volume so the patrons stay and the bar makes more money. Cool? If listening levels get too loud then people start leaving and the bar doesn’t make money. Did I mention that the bar needs to make money? If you don’t turn your amp down after being asked multiple times, and people start leaving because you are TOO LOUD, the bar doesn’t make money. Oh, did I mention the bar needs to make money?
Not only that the bar will have customers walk out because of the volume, the sound guy doesn’t have room to do anything in the mix. So your vocals aren’t coming through the mix and the crowd can’t hear anything you’re saying. Drums? What drums? The sound guy is trying to keep things comfortable, so he is not going to push everything else up to match the amp that’s too loud.
So, remember to be nice to your sound guy. If you help him, he will help you. And if you’re a dick, it looks like the suck knob is going to 11.