Johnny, as I indicated in my initial post in this thread I'd be skeptical that a cheap transformer oriented towards car audio, and perhaps even Carver's Z1 Coupler, would do justice to the OP's system, and to a JL sub in particular. As I'm sure you realize, all transformers are not created equal, and that is probably especially true in the deep bass region.
Also, are you sure that "any powered subwoofer that has speaker inputs has a similar transformer inside to pass the signal to the internal sub amp"? I have no particular knowledge of how that is generally done, but I would expect that in many or most cases it is done with resistors configured into a voltage divider circuit, not with transformers, resistors being a much less expensive approach compared to a quality transformer.
Regards,
-- Al