Don't know about the two center channel outputs, hadn't seen that before and from the little I know about two different speakers using the same channel can cause cancellation issues.
There are cables that can convert balanced to unbalanced and vice versa but since your subs don't have balanced inputs there's no point to using a balanced pre-out, just use the unbalanced preouts from your processor.
"I realize that somebody playing free music isn't as commercial as a hamburger stand. But is it because you can eat a hamburger and hold it in your hand and you can't do that with music? Is it too free to control?" - Don Van Vliet (aka Captain Beefheart) discussing commercial success in the music biz