From what I've read in the review of such "body-shaking" devices; They work best when crossedover (?) from the main Front L/R speakers. In another way, they don't just shake when there's a lot of bass present and you can also use one when you don't have the LFE channel present.

Try this site for reviews on "tactile sound transducers."
www.ecoustics.com

That being said, I don't quite understand your question. Where will this extra signal for your subs be coming from? If anything, it sounds like you would have less signal going to your subs. Maybe I'm just missing something.