Just a thought- I had an amp issue with my Hsu sub- a local repair shop(highly regarded) quoted me $445 for replacing the amp. I contacted Hsu and had a new one shipped to me for $250. I DON’T blame the shop- they’re in business to keep the lights on. It was pretty straightforward to replace.
My point is that most of these subs use a plate amp of some sort that is usually easily replaceable, so it might be worth checking out. Hsu is small fish vs B@W, so as we say, YMMV.