I did sound for a small festival this weekend and also played a short set with my duo partner before the headliner.
I figured that, since I'd already have my MG800, 2x12-3 rig (overkill for the duo, but they sound just as good quiet as they do loud) there, I'd make my sound gig easier by offering my rig to the opening band. That was fine, but, when they showed up, their bassist saw the size of the Genzler, and dismissed it as too small.
So... He pulls out his "superior" rig, of a GK 410 (I didn't notice the model, but it's one with the chrome grill.) "Passable," I thought. Then, he pulls his head out: a GK Backline. It was then I knew I'd be taking a DI before the amp. (When I'm doing sound, I try to either use a post-preamp DI, or mic cabs, because I know the frustration of having good gear, yet having it go to waste with a DI.)
So, I mixed their set and did ok, although he was too hot on stage, with no definition.
When I did my set, just to educate him, I muted my bass in the mains and monitors for our short sound check (they were still striking their gear) and opened my rig up for a bit, and said, "oops!" "I was still muted," loud enough for him to hear. The look on his face was priceless.
My professionalism returned, and I went back to my saved settings. I wouldn't have done it, but they weren't that easy to work with, especially for an opener.