Yes, Mark, that's about it; as long as a sub reaches low enough to excite a particular room resonance, it's the room, not the sub, which determines the best position. Although a corner is generally the best spot, in a situation like yours with a 13' square room, corner placement would result in exciting both 13' modes for length and width of the room. The result would be nasty resonances at about 43Hz(560/13)and the second harmonic at 86Hz. So, moving several feet out from the corner along one wall would help.
