When the receiver is
powered on, the hum's volume increases more. The receiver and the
subwoofer are configured to plug into different grounded outlets.
Both outlets are properly wired.

just sticking the 3 light tester into an outlet doesnt mean the GROUNDS are
connected correctly. in a perfect world, all the grounds only make contact
with the neutral at one point, and that is in the panel OUTSIDE YOUR HOUSE.
often electritians connect the neutral to the ground inside the panel, and
its also contacting inside the meter box. depending on how far this is and
other circumstances this alone can create a hum. and this is just one
example of where the tester says yes, but its not quite right. you may not
have proper ground rods installed, there may be a water meter that breakes
the electrical connection in the plumbing which was being used for the
ground. etc...

but i would first try this. disconnect any cable tv sources you have and
see if it goes away. if that doesnt work, do what robert bass said. either
isolate the ground plug, or put em both in the same exact outlet and see
what happens. you also didnt say if the different outlets are on the same
circuit..

lastly, before taking it back to the shop, take it down to bare bones, and i
mean bare. just plug in the sub, and use the radio on the receiver and
disconnect everything else. if this fixes it, bring it up one at a time
until the hum returns.

randy