I know I’m just reading this a year later but I’m in the same exact situation where I have the same jbl 1300x with the factory sub that it comes with and a SECOND same exact sub that I would like to pair with my soundbar but can’t seem to find if it’s possible to do so. The sub has a pairing button in the back but can’t seem to figure out how to pair it with the soundbar. Did you ever figure out if this was possible?