"Guest" says that festivals have little influence over sound levels.
I know of one festival that states that they would prefer artists to use their own equipment and professional sound engineers. But they add that if bands wish to use their own engineers "the quality and volumes at soundchecks and performances will be monitored by the festival's PA manager who will have ultimate control over such matters." This is because of the agreement with the local authority. It's a green field festival, using marquees.
So it is possible for festivals to influence sound levels.
You just have to make sure that you tell artists in advance and that your own equipment and PA people are as professional as possible.

I was at Shepley, stayed for a few songs from Seth, didn't think it excessively loud. I left for other reasons. I did notice that Seth's own PA person seemed to do a longish line check before he went on, presumably after a soundcheck before the concert started. seemed excessive. I didn't stay for 3 Daft Monkeys on Sunday, but suspect they might have been louder...

Incidentally, there seems to be an increase in people posting as "Guest" - I thought this wasn't allowed under the Mudcat rules.
   
Addison