This doesn't really help that much with your technical question, but I figured I'd give you a comment or two about the Peavey Tri-XXX and its tone. I run a Peavey Triple XXX Head through a Bogner Cab loaded with Vintage 30's, and the other guitar player for my current band has the same Marshall cab you have. Through either cab the head sounds beastly, especially if you're going for that sort of "Swedish" Metal sound (In Flames, Arch Enemy, etc.) My band plays in the style of In Flames / Children of Bodom and for our demo we're recording all rhythm and lead tones were recorded with the Peavey head and the Marshall cab. The results were awesome. We have some very heavy sounding stuff and the quality was more than I ever expected. But I will admit that in a life setup, the head seems to sound a lot better through the 4x12 with Vintage 30's. The tone is much more thick and heavy sounding, as well as there being quite a bit of low end. Just some thoughts.

What sort of guitar are you running? Do you use any pedals? What style of music do you play?