Have an SPL meter with you and keep it round the 95 - 105DB at the console and no one should complain about volume
After having mixed about 600 gigs with 87dB limit at the FOH on the cruise ships, I consider everything over 95dB is "loud"... I try to mix all the bands regardless of the venue size so that it doesn't get over 100dB at the FOH mixer at any point.
But, more pointers:
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Plexiglass for drummer: Sometimes a necessary evil, but I would recommend NOT getting them
at first. Or if you are thinking about it, I suggest you play at least one gig behind the kit with the plexi glass (with a floor wedge, not in-ears) and you will notice how awful it is. But I would still recommend getting them as the last resort if the drummers get constantly too loud. But start without them.
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Highpass filter: High pass EVERYTHING except the kick at 80hz (or higher, if it is an option) or have the subwoofers behind an aux send and only send kick (and subdrops) in there. Low end is what makes unnecessary loudness, takes too much headroom and makes everything sound smeared.
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More about mic choises:
* I talked about the drum mics already, so no need to recap that. Personally I prefer drum mics that don't need a stand, but use a clip instead.
* Vocal mics: There are better sounding mics than SM58's, but they are relatively cheap, sound decent, fairly insensitive to feedback, really durable and even the most snob tech guys will accept them (same with SM57's), so no real reason why not to get them, especially if the musicians are douche bags "rockstars" and don't treat your mics with the respect they deserve.
* Get a few (like 1-4 pairs, even cheap ones will do) SDC condenser mics for stuff like percussions. If you want, you can get 1-2 LDC mics, but in general I suggest to stay away from them as they are usually too sensitive for feedback.
* DI-boxes: You also need DI-boxes. I would say you need minimum of 4, but I suggest getting 8-12. Get like 4-8 good ones (Radial, Countryman, BSS...) and then fill the rest of the amount with Behringer DI-100s (they are like what, 29€ a piece?), so that the fuckers can't complain you don't have enough DI-boxes. That amount will do for almost everything you will ever encounter, and if it doesn't, tell them to bring their own DI's.
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SPL meter: Get one, I think you can get them fairly cheap from Radio Shack. Within 1-10 gigs you will notice what is considered loud and what is considered quiet by you and the staff, and try to keep it at the edge. Remember that you use a gate to make something quieter and a compressor to make something louder, so thats why I would suggest not to use a master compressor. But bus/group compressor? Definitely maybe.