Spooky Radio

A few days ago, the sub-woofer on my '97 Expedition w. Mach radio system, began to rumble and increased in volume regardless of radio setting (including off), until it reached max output. The sub continued to rumble at max volume even after the vehicle was shut off. :screwy: I had to remove the fuse to shut off power. Has anyone experienced this? Would this be a bad sub-woofer amp, a short in the radio control head, or both?