Yeah, Nick; theoretically the ceiling should also be taken into account, but it usually isn't considered, since it would be very unusual to have a placement where the distance of the woofer cone to the ceiling was identical to the distance to the floor and both walls.

No, the evenly divisible lengths factor applies to the size of the room and the resulting modes. The reflections from the nearest room surfaces are a completely separate effect.

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Enjoy the music, not the equipment.