John, I'm also puzzled by what you report. My understanding is also that imaging depends on midrange and upper frequencies and that strengthening response in the mains below 80Hz or so by setting them large can't affect it significantly since those frequencies are essentially omni-directional. Theory aside, in a system where I use a sub I always set all speakers "small" and it sounds a bit better(no change in imaging, however).

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Enjoy the music, not the equipment.