Richard, you hit it right on the head! The audience is the problem! You get the sound balance perfect at the sound check in the empty hall, then they let in several hundred 200 lb water-filled sound absorbers and it takes you about three or four songs (on the first song you are also having to pay close attention to performers who are frantically gesturing at you to turn them or their instrument up/down in the monitor) to reset the EQ and balance levels. The only good thing is that there are no more people on the stage than at the sound check so the monitor mix changes are usually minimal. Polite performers will end a song before asking for less/more volume/mids/bass etc.