This isn't going to help, but since I experienced the hum similarly and decided to live with it, after a couple of months the hum became intermitent. After I gave the subs away, they hum never occurred in my son's homes. I've since had one back here for a shootout...no hum.
Methost, I understand your disappointment. If you keep it, enjoy it for it's output and see if, like me, it will just go away.