Your sub's power is probably on and you're not sending it a signal. Perhaps your sub interconnect is picking up some noise? Have you tried other cables to make sure you didn't damage the wire (or connector) during your receiver swap? Also make sure the cable is not near any power cords.
Is the humming noise coming from the driver or inside the sub (i.e. the transformer for the amp)?
A cheater plug may eliminate the hum, but you would be creating a serious potential safety hazard so I would not recommend it as a permanent solution. The ground is there as a safety precaution and defeating it, defeats the safety. If you can't get rid of the problem after checking all patch cords and figuring out which component is the problem, there are various hum elimination products. You could go to a site like www.markertek.com and enter the word "hum" and do a search. So the first step is isolating the problem and then perhaps reposting so we can come up with potential solutions.
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