I was standing by the desk at Shepherds Bush Empire, many years ago, at a Little Feat gig, featuring Craig Fuller. That's how long ago. They started and the sound was perfect. Clear, well balanced, well defined and a good volume.
I watched the FOH guy spend the whole gig "operating" the desk. He never stopped fiddling and the sound went from just like the record to and awful indistinct mush at about 3 times bearable volume. If those guys can't get a sound man who knows what he is doing then what hope is there for anyone else.