Its a tricky question.

I can think of groups who insist on using monitors, and some peopele think they are deceptive - of whats going on out front.

Basically all these small systems - and probably all systems - have an optimum level they can function at. The big problem of course is that when the noise level of the venue goes up - people start looking for their cues,, and they can't hear them so well. so they start mucking about with the basic sound that you've worked so hard to put together. They start turning things up and speakers round, and the audience who are actually listening can start getting their ears battered.

If you're really worried, its as well to use the unbalanced line inputs for the mics - they will give you more headroom. if you get a harsh sound when you take a volume up - the first villain in the piece can be the hi frequency parametric control. don't get married to any reverb setting - every space, and every audience probably dictates some adjustment.

all the very best of luck with your gig. the monitor output is puzzling - play a cd (with the fader down and then up) and try a jack out to your bass amp - see if anything comes out.