Here was the reply from BIC:
The first thing to do is determine whether the sub's amplifier or its woofer is responsible for the noises. To do so, remove the amplifier from the cabinet by taking out the screws around the perimeter of the black control panel. The amplifier is built on the inside of that panel. When you have access to the woofer, reach in and wiggle the lead wire clips off the woofer's input terminals.
Then, turn on the system and play something at a reasonable audio level. Remove the speaker cable from one of the front main speakers, being carefull not to let the bare wire ends touch together. Hold those wire ends temporarily but firmly to the woofer input terminals. If the woofer plays, even though a bit muffled, and you don't hear the crackling noise, it's OK. In that case the amplifier is at fault. If you hear a scraping noise or distortion with the crackling noise, the woofer need to be replaced and the amplifier is OK.
When you know which is causing the problem, wrap that component carefully in a couple of layers of bubble wrap and put it securely in a sturdy carton, but don't seal the carton yet. Put a note in the carton with a brief description of the problem, and your name, address, and telephone number. Also, put in a copy of your sales receipt showing the date of purchase and also a copy of this correspondence. These are essential.
I'm sharing it here because someone else out there might benefit from using this technique to isolate a fault in a sub, but also to indicate that BIC was extremely responsive to my email and provided an intelligent approach to diagnosing the problem, as well as a way to avoid shipping a very heavy unit all in one piece. So so far I am having a good experience with them.