but now that they are all polarized you can't do that...

...replace the plug or file that prong down.

Does your stereo have a ground pin on the plug?

If so check that you house is grounded correctly. Lots of times ground wires get corroded and nobody cares until the stereo starts acting up. Go to your breaker box and follow the ground lead from there to the actual ground which may be a water pipe or a rod buried in the ground. Check all connections and replace any that are corrioded.

Make sure that all equipment that is connected to the stereo is correctly grounded and goes to the one ground.

Or you might need to relace the smoothing capacitors in your power supply.