Patrick,
Hope the gig is a smashing success. I can totally relate to your situation...it happens more often than not, at least in my area. From what I see where I live....
1. Other than weddings, corporate gigs, and big A list shows, there is is little to no money in live performance. The wedding and corporate gigs aren't the "art" most artists desire and people playing A list shows aren't reading the LAB Lounge. What's the old joke - a Jazz musician is someone who takes a $5k instrument in a $500 car 50 miles to play a gig for $5. Sadly this is much closer to the truth than most realize.
2. The band is often clueless about live sound and treats the PA as an afterthought. Hiring a sound guy isn't even on their radar. They are only aware of the sound on the stage and the reaction from their fanboy friends. If they only knew what they sounded like in the audience....
Many times I've been a hired gun (guitarist) and found myself pushing to insure the sound is acceptable. I'm lucky in that I run a recording studio, so I guess that gives me credibility when I advise them on live sound. Still it can be an uphill battle.
I've also found things can change dramatically once you arrive at the gig. Despite assurances that the venue was informed and on board with the plan, you may find someone you didn't even know existed has (or thinks they have) authority and insists on a setup that makes no sense.
I guess my best advise is to be ready for anything, and know when to just shut up and play your instrument. You can lead a horse to water......