Unplug the sub cable at the sub. If it still thumps, the spirits are coming in from the AC plug that your sub is plugged into. When you plug/unplug it, it's creating a low frequency disturbance on the AC line that is getting through the sub's surge protection circuitry (which is designed for high frequency disturbances only - like lightning).

If it doesn't thump, then your cable is picking up a signal when you plug/unplug your VP50pro.

The solution is to stop messing with your VP50pro's power plug \:\) .

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated