This could be DC on the mains. I had exactly what you are describing happen in my setup roughly 2 years ago.
It happened whenever my wife would turn on a box fan we had in the window - as it was in another room, and seemed completely random, it took awhile to figure out the cause. It would send about 100mV of DC over the AC mains (that registered and disappeared so quick on my spare meter, you'd think it was just a fluctuation on your DMM) and I would get a nice quick thump through the driver. Most people think Surge protectors and power conditioners filter out DC, because they will list AC Filtering, but 99.99% of them don't filter out DC.
I'd start by removing the RCA cable at the back of the subwoofer. If it still happens, with no connection from upstream and the subwoofer on, my inclination would be DC.
If DC, you'll need to find out what is causing this (usually equipment with a motor that kicks on - fridge, compressor, hair dryers, etc.) and replace it or depending upon the price of the equipment (I had a customer's expensive meat freezer do the same), buy a DC filter.
Needless to say, I bought a new box fan for ~$30 as it was cheaper than the alternative.