A word of advice - if you've only ever played at rehearsals so far - take a "sound engineer", otherwise known as a friend who knows their way around what music is supposed to sound like out front - not necessarily know how to operate a PA - and will do it for the price of a pint or two

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It's really important to listen to, and act on, whatever they say about the sound - you can't hear it clearly yourselves from on-stage, and it's easy to get a bad balance that sounds good on stage but dreadful from out front. If the "sound engineer" says someone is too loud, or too bright, or too muddy, or whatever, don't get offended and stick to your guns... they're right. It's no good having a "great guitar tone" from where you're standing if it's overpoweringly loud and bright out front and detracts from the sound of the band, especially the vocals. To the punters, vocals are the most important part of the music.
Quality is far more important than volume. Get the best sound you can and then make it as loud as necessary, not the loudest sound you can and then try to deal with feedback or mix problems. Sometimes it can be difficult to achieve relative to the drum volume, but it's still better to leave the drums slightly too loud than make everything else so. (Which will probably just encourage the drummer to get louder anyway

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Make sure the bass amp is firmly on the floor, not one of those tilt-back ones (unless it's also going through the PA, which I'm assuming it won't be), the guitar amps are *not* on the floor - they should be up at least the height of a beer crate, preferably two, unless they're 4x12"s in which case castor height is (just) enough, and that the vocal PA cabs are *above* head height for the audience. You will get a far better mix with those positions before you even start adjusting anything.
Apologies if you know all this already... but it's entirely obvious from experience that most bands don't, including a lot that should, given how long they've been playing.