Had a vary similar problem with one of my powered subs. Turned out to be a ground loop problem.
Google "ground loop"--you will find entire sites devoted to tracing ground loop hums.
Try a 3 prong to 2 prong AC plug adaptor, or plugging the sub into a different outlet.
Disclaimer: Use of these adapters may not comply with your local codes. You Insurance company may not be happy, or somebody could get shocked. Use at your own risk.