some venues these days have their own backline, but this is a problem, i hate when other bassists mess about with your settings, im not anal, but graphic eq is something i just dont want to remember when ive got it right. maybe if your the headline band, insist the venue find an amp for the supports if your worried, or just ask the other bands if they have a spare second amp
i doubt the lecturing will help though, if they are stupid enough to do not know they own an active instrument then your not going to get through, and people who do know how to use the gear will think your a t*t