I found this discussion when researching the identical problem reported by others. In my case, it was down to the cheesy power cord. I thank those who pointed me in that direction.
I got a pair of needle-nose pliers, bent each of the connector blades on the back of the box inward 1/32 inch, and reconnected. The plug now requires a little effort to push on or pull off, and is non-wobbly when in place. The sub was fully functional when I repowered, and it has stayed on.