Before we get into a whole bunch of diagnostics. Some questions;
A. When you say a buzzing, is this a kind of hi pitched buzzing noise, or more of a hum?
B. When you are very close to the unit, does the noise sound like it is coming more from the back of the unit. Or is it a hum that you hear from the woofer, and if it is more of a hum, is it effected by increasing or decreasing the volume control on the sub.
The buzzing if this is what it is, and it that sounds like it is coming more from the rear of the sub could be a noisy transfromer in the sub power supply. The fix for that would be to call Joe in service and arrange to either send it back or have Joe send you a new part and you would return the defective one after you replace the unit.