That's 11, my friend. Good topic, though.
Ted Nugent and Motorhead can also lay claim to that throne.
Loudest band I've ever seen is Cannibal Corpse...and I've seen Motorhead, too. Not in their prime, so it may not be fair to rate them below Cannibal Corpse based on that experience.
I saw Slayer is San Diego in 2003. They played to 10,000 people. I saw Metallica play to 60,000 people in Toronto the same year. Per square meter, Slayer was louder.
Can't forget Kiss at the Tacoma Dome in 1996 either. My ears rang for days.
Local bands-Suffocated, AK-47 and Raging Mob all take it to 11. The Reason can take years off your eardrums in a bar setting, but come off pretty limp in a large venue.