HELP! Connecting JCM800 4104 to Marshall MX212R Cabinet

[El Gringo]

This probably sounds dumb, but are you sure that you're using a real speaker cable and not an instrument cable?

Not sure if an instrument cable used as a speaker cable would cause your problem but it's definitely not something anybody should do.

Sometimes an overlooked detail for those not in the know . You guys will laugh at this but with Marshall's stacks there is some thinking involved versus plug in and play with my old Fender combos which tone wise sounded like ass . Seeing now how I know what real tone should sound like !

 

[Georgiatec]

Confirm your cab is what is on the label. If it is then you should be good to go with 2 x 8ohm loads equalling 4 ohm total load. The other cause could be a dodgy impedance selector switch.