You have mixers because ? ? ? the client inherited them? Well, I find it pretty effective to sell Prontoid remotes by having the customer use all six remotes while telling them that there IS a solution, and though it will cost bla bla bla...

Does he need mixers because he is using several microphones at a time? Or is he saddled with mixers because that is what is already there? You have to figure out how difficult it is to work the system and propose an easier way. The easier way will cost money but will solve headaches. No money spent now MIGHT mean that the SuperBowl party is a bust because someone just did not GET that they could blow the amps if they did such-and-such...and how much will THAT cost, not just in money but perhaps reputation? If a customer leaves the club because of the sound system, are they likely to come back in a week?

Get your ducks in a row. Get a price on a preamp to use in the system. You know, ONE volume control!
I have a buddy who does lots of clubs, so knows the kind of equipment to use where nearly brain-dead employees will still not start fires or spend the night in silence. It is out there; email me with some particulars if you want some particular advice.