[font=trebuchet ms]I'm not familiar with Marshalls so you may want another opinion on this.

With your cab set up as a 16 Ohm load, I think your amp output should also be set at 16 Ohms.

But even that kind of mismatch would be unlikely to cause overheating as severe as you describe. It sounds to me like there is a problem with the bias circuit for the power tubes. With not enough negative voltage applied to the grids, (like if the bias supply cap starts to short out), the tubes will flow far too much current and overheat. This is a job for a competent tech to handle.[/font]