PSW111 Buzz/Static

I think this was/is a somewhat common issue with this sub but I haven't been able to track down a specific answer. I also tried emailing Polk customer service but haven't heard back.

A few months ago I noticed a static/buzzing sound coming my sub...it was in 'Auto' mode and all of my other components were off. I would hear the same thing every few days but then it became more and more frequent, to the point where now there is rarely a time when it doesn't happen.

I've kept an eye on it and tested a few things... It's completely isolated to the sub itself, the static/buzz happens even when the audio cable is disconnected. It happens in both 'On' and 'Auto' mode, although sometimes it will stop temporarily if I toggle from one to the other. I don't hear any static or buzzing when it's in 'Standby' mode. It does not increase or decrease with the volume, and the Phase switch has no effect either way. There's also no correlation with any dials or switches (not a potentiometer prob).

The bass quality itself is also affected. At times there is no output at all, and other times it is very loose and muffled with intermittent static. There is no physical damage, it's in perfect shape, and no changes have been made anywhere in my HT system for at least a couple years...had the sub itself for six.

From what I can tell it seems like it's an amp issue, but that's about as far as my technical knowledge reaches. Can you help me narrow this down? If it's a capacitor that needs upgrading or something, I have no problem handling it myself if someone could just point out the most likely culprit(s).

What do you think?