Sound like you've blown your speaker cab? Before going out to buy new speakers for your 4x12 Marshall cab, be sure to check to see if the buzzing is not being caused by the handles. Believe it or not, a badly buzzing handle can sound exactly like a blown speaker. Marshall used rivets to fasten the handles to the cabinets, and often the handles are loose. Replace the rivets with screws or with bolts and T-nuts.
To check the cabinet. have someone pull on the handles while you're playing to see if the buzzing goes away. I used to hang my gig bag from one of the handles on live gigs to prevent it from rattling.