According to Metal Blade, Manowar has to be the world's loudest band:
"In Hanover, Germany they established a new standard in ear splitting power by breaking their Guinness record for loudest band in the world. Two sound specialists officiated, measuring and documenting with painstaking care as Manowar shook the city, playing live at a staggering 129.5 decibels through 10 tons of amplifiers and speakers measuring 40 feet in length and 21 feet in height."
As far as I know, this was achieved in a rehearsal with the band and technicians wearing protective earplugs. Had they played that loud in front of a real audience, some heads would have been blown up.