Sean, mid-wall placement uses the two techniques for lessening the effects of room modes(placement at points where the mode is at a minimum or placement where they're opposite in polarity and cancel)in combination. For example, mid side walls are at length mode minima and width mode maxima(but the opposite side wall is at the opposite width mode polarity). Corner placement is at all mode maximum points and uses the fact that the longitudinally and/or diagonally opposite sub is at a point where the mode is opposite in polarity to cancel the effects.
The two placements are similarly effective as to evenness of response, but the corner placement leads to a higher sound level per sub being realized. Note again the comments from pp. 226-227 of the book where Dr. Toole states that a two mid-wall placement with two subs co-located at each spot(and therefore strengthening the mid-wall output)was by a narrow margin the best of the options for a four sub placement.
