With your mains and surrounds set to large, none of the information from those channels is being redirected anywhere. With the sub set to "on" all of the LFE remains there. With the center set to "small" and the crossover at 80Hz, anything below 80Hz is redirected to the sub, where it joins the LFE (if there is an LFE track). Is there a problem with this arrangement? I'm not sure I see a reason to redirect everything below 250Hz to the sub (or even a method for doing so, short of setting all channels to small and raising the crossover), and I can't imagine how activating nonexistent surround back speakers with a high crossover would help - it would deprive you of some of the surround information (since it would be removed from those channels to generate a signal for the surround back), and the amount of bass in the resulting surround back signal isn't likely to increase the overall sub signal appreciably. If you were trying to get more bass, it would be easier to adjust the trim on the subwoofer channel (or at the sub's amp) up a notch or two.