The Crutchfield figure is somewhat outdated. In original Dolby Prologic, the center channel was a matrix of the Left and Right. Accordingly, alot of info was coming through the center. With Neo6 or any of the new decoders, you can still get alot of center info.

With DD or DTS, there is a discrete center channel. Accordingly, what comes out of the center is whatever the mix puts into it.

To answer your question, I have a separate sub in my center channel wired just how you describe. I set the center to large. And it works great. Takes some fiddling to get it to blend correctly with the center.