Charles, of course that two mid-wall plus co-location option can use either the side walls or the front and back wall.

The co-located subs can either be stacked on top of each other(presumably better with the horizontals)or placed immediately beside each other(better with the verticals). Note that the strongest mutual support would be if they were turned so that the drivers faced each other and were maybe just 2" apart.

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Enjoy the music, not the equipment.