I don’t know much about the Bose system but I read somewhere, on this forum, that the input signals to the Bose amp are all digital, and that only the outputs to the speakers are analog. I would use the factory subwoofer speaker signal and feed it into either a hi/low converter, or use your new amp’s speaker level input, is so equipped. Using a single X1 cable harness might make it easier to access the wiring, unless you pull the entire rear panel off. The JL Audio package looks good. I wound up installing a single 10” subwoofer between my 2nd row seats. Good luck and let us know how it goes.