All you need to worry about is the sound on stage. There's not much you can do about the sound in the front of the house without your own soundman. You'll have to trust that this guy is good enough, to realize how to mix your band right.
The main thing is that you get your sound on stage right. That everything each of you wants in your monitors is there, and at a comfortable volume. Tell your bandmates NOT to be intimidated, and afraid to say if something is not right. Until they have been around the block a time or two, some guys just clam up and accept whatever happens, only to b*tch about it later.
If you need the vocalist louder in the stage-right amp, then tell the soundman this. If your singer wants more drums in his amp, but less back-up vocals, make sure he tells the soundman.
If I am working with a soundman for the first time, and he doesn't really know the music, I prefer to get a good sound check, even if it means sacrificing one of the songs. When you hear yourselves onstage well-mixed, and this makes you more comfortable, it will come through the music. Everyone will play better, and consequently sound better, with fewer mistakes.
And whatever you do, ANYWHERE YOU PLAY:
The sound man is your friend. Be disrespectful to him, or unecessarily demanding and you are F*CKED for as long as your set lasts. And don't think these guys won't pay your band back by screwing your sound! THEY HAVE THE POWER! Don't let ANY of your band members make that mistake. I have gone so far as to tip the soundmen before a show, and if I can't do that, I will get the soundman's name before the show, and say something like "Before I introduce the band, everybody give Brian over there, a big hand. He does a great job on the sound, and he's the one making ALL the bands sound good today! Give it up for Brian!"
Do this, and I guarantee you, your soundman will do the best he is capable of, because you have just told your crowd that he is the one making the bands sound good, so by default, if YOUR BAND doesn't sound good, they know who's fault it is. He won't embarass himself like that. He'll pay as much attention to your sound as the headliner's after a set-up like that!
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