If you connected as Gonk suggested, I would check your video/audio input assignments via the 990's menu.

Your hum could be a ground loop, so you have some detective work ahead of you. If not already done, you can try plugging your gear so everything comes off the same breaker.

If none of the other speakers produce humm, I would also try swapping the subwoofer's cable to see if that's the culprit. Don't forget to unplug your sub when you do that (been there, done that).