Are you listening with the mains on? If so you should hear the difference since you are adding much less delay as you increase distance. If your room is lively (ie very resonant)you will have a more difficult time hearing the best distance setting. As I recall from your pictures, you have bare walls in your HT so I suspect you are getting alot of LF reflections and standing waves in room. It will be more subtle, but there should be a setting where drums sound snappier and bass guitar notes decay more quickly. If you still can't tell don't worry about it and just leave it at the normal plus 3ft distance. It likely indicates a need for bass traps/room treatments.
