How have we determined that it’s the speaker and not the amplifier plate?
I’m just gonna guess here but you’re going to be throwing good money at something that really can’t be fixed. These types of products tend to not have aftermarket parts available for them. If you can contact the manufacturer and they can send you out a subwoofer then you might be in business. You would think you could get some action on a $1200 product but that’s the way they do business today.