Marshall's can be a beast that's hard to tame, but once it's there, you can't beat 'em.

I'm not that familiar with that particular model, I play a JCM 800 and JCM 900, but I'd suggest tweaking the EQ while you have it cranked up. The EQ settings can make a difference at different volumes, especially on a Marshall. In other words, if you have it sounding great at say 3, then crank it up to 8 or 9 (the "sweet" spot on Marshall's), you'll probably have to adjust the EQ to compensate for the added tone from pushing the tubes harder. At least that's how it is with my Marshalls....