Jams and earplugs:- earplugs are no good, if the noise whites-out to earplug level it is way too loud. If you use 'plugs below that you cannot hear yourself letalone the other players. PA is for vocals and harmonica/sax/etc only, no mic'd amps (gets too loud front of house). Amps 17W to 150W but balanced to the little 'uns: do not allow players to bring their own amps (unless they're a bit special): the "resident" band provides enough interesting ones (but sometimes hogs them). Sometimes loud is good, but a jam needs you to hear everyone in the, erm, "band", train wrecks are optional. Rotate players ~2 numbers each on semi-random selection. Dunno how small that Cornell was but they don't usually need mic'ing for 100 punters. Circuit that dives amps at lower output is called a master volume pot.