I have a '70's Silverface Bassman and there's this toggle switch on the back panel labeled "Ground." You push it in and you can adjust it to three different positions- left, right, or middle. I am completely lost as to what it does.
It does exactly what it says, it selects the right ground for the amp. If you've got the wrong one selected you should notice a hum coming from the amp, and you might even get a little zap if you touch something that is grounded while you're playing.