Try using a cheater plug, most of the time that is the cause of the hum.
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Plugs are grounded to avoid manslaughter charges when the electronics short out inside the component, you touch the unit, and the total output of the Hoover Dam momentarily passes through you on its way to ground. While it's not really that dramatic, next time you're out take a look at the footwear of the musicians down at the local hotspot. They know the benefits of rubber soles.
Manufacturers who stuff digital processors into their units are particularly adamant about using the ground, but the units seem to work ok without it. If it helps, audio hobbyists commonly believe that if one component is properly grounded, the rest of them will be too through the interconnects.