If the mains are set to large, the default is for the sub to be set to LFE (at which point it'll get the LFE track on 5.1 sources as well as bass redirected from small speakers in the system). The other option for the sub is L/R+Sub, which does the same thing except it sends a copy of the left and right channels below the left/right crossover to the sub (so you get that bass signal twice).