I used to play in a band where both guitarists used 200w Blackstar half stacks, even in smallish pub gigs. Their response when I pointed out that they were way too loud was "you need to get a bigger amp" My answer to that was "why?, so we can deafen the audience as well as ourselves" I didn't hang around too long, I decided I liked listening to music too much, and wanted to be able to do so til my old age.... In my experience, the band being too loud on stage is the bane of any sound engineers life. As for the OP, a quiet, polite offer of help would probably have been ok, provided the band weren't complete knuckle-draggers...