Bass modes and nodes will be affected by the subs location (S) throughout the room. The shape of the room will also affect the sound. This is not necessarily an argument against placing the subs higher up though. Most of the reason you see subs on the ground is that subs are heavy, and it is it less visually obtrusive on the floor.
Your AVR uses top spec Audyssey Mult EQ XT32, which is a pretty powerful tool. If you try this method, I'd try placing both of the left hand subs up high on that ledge, and then let the room eq adjust for the two groups (the ground level fronts, and the raised subs), so that Audyssey can adjust the phase, level, and distance settings for the groups based off of similar placement.
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