(Brian) writes:

When I plug my subwoofer into my receiver and turn on the receiver, I
hear a hum.

The receiver and the subwoofer are configured to plug into different
grounded outlets. Both outlets are properly wired.

Other than upgrading some RCA cables and changing the
outlets into which some components plug, no changes were made to the
system.

You appear to have every symptom of a ground loop problem. If
possible, plug the receiver and subwoofer into the same outlet, and
I'll betcha a quarter the issue goes away. :-)

If plugging them into the same outlet is not feasible, seek out an
audio isolation transformer for the line level signal that feeds your
subwoofer. This isolates the grounds of the two affected component
and often eliminates the hum.

Best Regards,
--
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