Were they in the same PLANE? Did you mount the box speakers so that their fronts were exactly the same distance from every wall (including the mounting wall) as the others? It's not just "what's coming out of the front of the speakers" that matters; it's the room reflections, too. And "what's coming out the front" is certainly affected by the size and shape of the immediately adjacent surface.
Look, I don't know why you perceived comb filtering, but you're going to have a hard time convincing me that either the room is irrelevant or that your conditions were sufficiently controlled as to be conclusive.
