Sorry to be so slow responding
The loudest gigs I ever went to were Motorhead at Hammersmith Odeon Merry Bastard Christmas tour in about 1989, and Nuclear Assault in a venue holding 400 (where they used the same set up they had the night before in a venue for 4000).
Both were mind-blowingly loud. A lot of people do wear ear protection. I have personally never bothered. Your ears buzz for a day or so, but that's it.
For festivals, sometimes I am not sure the outside actually helps this type of music. Slayer at Sonisphere were disappointingly quiet as the sound all went up. In a venue with a roof they are usually $#@!ing LOUD as well.
Saxon were a very loud band back in the day. Maiden were loud around 1988 - 1990. When I have seen them since they haven't been up there for volume.
Prodigy are very loud too thinking about it. Metallica can be. They certainly were on the Damaged Justice tour.
For Bloodstock, the main rig is pretty substantial and with it being only about 10-12,000 people there it is easy to get pretty close to the front so the volume can be excellent.
My idea of a good volume level is when the drummer hits the bass pedals and it feels so powerful against your gut that you almost $#@! yourself. That means it is just about loud enough.