I have a red stripe Mark III head and..... it's quite possibly one of the loudest amps I've ever heard in my life. No matter what I'm doing, I never feel like I'm even close to not having enough volume. It's just insanely loud.
Anyway, to answer the question, the loudest I've ever needed this amp to be was on.... maybe three.... MAYBE four (master volume, I'm talking). Anything beyond that is literally ridiculous. This is through my Mesa 1x12 cab and set on 50 watts. I can only imagine this amp set on 100 watts and through a Mesa 4x12. I mean, ****! LOL!