I've got a hum in my Sunfire sub.....

Theo is on the right track if you have the money for the upgrade. Otherwise, Sunfire will only charge you shipping to recap/repair/check-out the unit. Many of the Sunfire's have hum problems and the company is great about helping to cure it. I sent mine back, they did a lot of work and only charged me shipping!

...it turned out to be my amp for HD OTA signal. As soon as I unplugged it, it stopped. I'm not sure what to do now though. I love my HD.

Ground loop isolater in the coax line to/from your HD signal.