Should hook I speaker level sub to center or sides

I have 2 Avantgarde Duo woofers that I don't use since I built basshorns. I want to hook them up as subs to my theater but not sure if they should be hooked to the center channel in mono or to the front sides.

Can't use line level sub out as the speakers don't have a line in.

I'm thinking mono to the center but not sure. I know,, try it, but easier just to ask

thanks