Mtrot...I think what D Edwards is getting at is that what your focusing on is just a temporary solution to a situation that should really bring the best out of your setup. This is part of the reason why attention should be paid to speaker selection.
Setting the center to small may work fine for you for now, but once seen and heard how sub intergration really effects the overall performance..if you haven't already, there's a good chance you gonna want to change. If you fix things early instead of slapping a band-aid on it, you're gonna be much better off.
Since there are certain limitations holding you back, I can see how you've focused on just the settings and not the overall setup. If the band-aid approach works for now, although it wouldn't be my first choice of approaches, then do what works for you. But, keep in mind you will be selling you system short though.
"But they do shake the room pretty good if you turn it up. "
I had the same misconseption about a sub and/or front-sub combo system. I was close to spending the $2500 for a Def Tec setup. Glad I didn't. Room shake is just a added bonus to a strong sub. However, it really isn't the main point though. It bares more weight and has more effect on the entire system than you think.