We lost power this morning due to a thunderstorm, and when the power returned, my trusty old sub had a very loud hum.. I changed power source, no luck. Preamp and amp both work fine. Any suggestions?

Did you try a different input cable? Or, another pre-out from the Preamp? I would try the center channel pre-out for a test to rule out the sub out from the preamp. if another cable and different pre-out produce the same loud hum, then you should contact Outlaw for assistance.