Nick, as you say, for the best effect in minimizing room modes, placement at the midpoints of opposite walls(or in diagonally opposite corners)is best. If the subs must both be placed along the front wall, the 1/4 placing shown is correct. As discussed by Dr. Toole and others, that placement serves to minimize or cancel some modes related to the width of the room. It has no effect on the modes related to room length, and doesn't have all the benefits of the placements described above, but is worthwhile.
