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

....I've tried changing cables and plugs. I'm running it with the RX-Z9. Any thoughts? It's a hum that there when the RCA in is plugged in off of the front sub out.
I had a boston acoustics sub once that would hum if I bumped the RCA cable end hooked into the sub itself. Bumping it again or shaking it would make it good with the world again. I wish I had a more scientific reply, but you never know.
Try floating the ground pin of the powercord.
Bad solder joint on ground on front RCA jack?
try buying a grounding plug on the end that converts the 3 pin from the sub to a 2 pin for your wall. Often the ground pin causes the buzz.

Could be caused by a ground loop. That is, if you have the rest of the system plugged into one set of outlets and the subwoofer on the other side of the room into a different set of outlets, you can have a difference in ground potential. The end result is an increased noise floor and / or the potential for hum. You may also have a problem with one or more components plugged in with the incorrect AC polarity. This too will result in an increased noise floor and / or the potential for hum.

Other than that, i've seen more than a few comments pertaining to hum related issues with Sunfire subs. Call up Sunfire and ask for Robert in technical support. He's VERY thorough and will help you as best possible over the phone. Sean

PS... Any time you call a manufacturer pertaining to one of their products, have the model and serial number handy. This allows them to check and see if your model was produced before or after specific production changes were made, etc...
Those suggesting a ground loop are probably correct IF the hum is there when the sub is plugged in to another sub out connection as well. I interpreted your description as saying it was ONLY present when plugged in to the front sub out. Most HT recievers have a rear sub out as well. If my reading was incorrect, then I vote with the ground loop group ;~)
Mine doesn't know the words either! Actually a buddy of mine has one and it was doing the same thing. He contacted Sunfire they have a flat fee of 250 or 295 I forget, but they will repair it and in his case they put in new updated amplifier. He say's it performs better than it ever did.
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.