What I was hoping ... that I could hook the rear outputs up to the sub input, ... But I suspect that I would only be able to use the fade then to control the sub

That is correct -- but what is wrong with that? That is a great way to go, otherwise you have to crawl around in your trunk or under your seat or wherever it is you have your amp in order to change the sub output relative to the rest of the system. I'd do it.

tanstaafl.