After moving some furniture around in our living room, we had to move our Kube 2 subwoofer to a new location. After re-installation and power up I discovered a buzzing noise. Due to lack of sockets in its old location it hadn't been used in a few months so I was very disappointed. The buzzing only appeared when the input cable from my Onkyo av amplifier was plugged in, however wiggling it about occasionally stopped the buzzing. I initially thought my subwoofer cable was at fault so I swapped it out for another shorter cable and the buzzing returned - both phono sockets on the sub appeared to be affected so it was either something inside the sub, or some kind of grounding problem.
The noise sounded similar to the buzzing I used to get with a pair of turntables if they weren't grounded to my mixing deck, so after doing some searching online I found a few solutions ranging from removing the earth connection from the plug (don't do this), or purchasing a subwoofer cable with included earthing strap (upwards of £30).
To be sure, I connected one of the testing probes of my multimeter to one of the screws on the subwoofer and the other probe to one of the screws on the AV amplifier, the buzzing immediately disappeared.
My old turntable came with crimped terminal spades which attached to a screw earth connector - or in the case of my old amplifier, one of the chassis screws. So I made up a short length of cable with crimped terminal spades on either end and fastened the cable between the two screws.
I'm guessing wiggling the input cables around occasionally completed a ground circuit between the chassis of the sub and amplifier via the metal phono connectors.