Another thing to try after going through all the procedures above is to get some wire and try grounding the chassis together in various arrangements and also to a known ground such as a screw on an AC outlet. Oftentimes your components can be at different ground potentials and you are hearing the residual 60Hz hum as well as RF and EMI traveling in ground loops. If you get everything grounded to the lowest potential you may eliminate the hum. I have done this many times myself. I would use a 3 prong to 2 prong cheater plug as a last resort, as you can sometimes incur a dangerous shock situation. This happened to a friend of mine and it scared the bejabbers out of him.
