I was in a pub in a remote village on the north west coast of Scotland a few years ago. There was a band playing who'd come up from Glasgow (acoustic guitar, fiddle, bodhran and vox through a small PA) and when they took a break, the fiddle player did a short set of Brassens, Thackray etc. chansons, accompanying himself on guitar. The sound was so bad - guitar too loud and boomy, vocals almost inaudible - that after the first song, since his band mates weren't going to do anything about it, I was compelled to rush over to the stage area, tell him the sound was awful and then make suitable adjustments to the mixer/amp. Fortunately my guerilla sound engineering was gratefully received on this occasion, but I do appreciate that in other circumstances it might not have gone down so well!