I have an older Marshall JCM 900 how can i make it sound like a krankenstein?

I use a boss metal zone on clean, usually swept mids. Just cannot get the sound i want out of it. I was wondering if the tubes could make the difference from British mud to American metal. I have not changed the tubes in seven years.I have an older Marshall JCM 900 how can i make it sound like a krankenstein?
I don,t believe 900's were meant for metal. They can excel at 70's- 90's rock in roll. Also the ones with the el34 tubes sound way better... and change and re-bias your tubes once a year if you play a lot.