We have found in a couple folk venues that even though we ask the sound guy to keep levels lower and the treble turned down that he keeps messing with it during the set. It seems like he looks for the quietest songs/tunes to push the "suck" button, and annoying feedback results. I believe that is due to the fellow's high end hearing being damaged, so he tweaks the treble till he can hear it and then all hell breaks loose.