Dave, my guess is it has to do with the location of the rear Q's compared to the side Q's. Also, the room characteristics may be playing a role. If I remember, your ceiling is vaulted and bass frequencies like to collect in corners where two walls meet. Also, aren't they higher than the side surrounds? Also, the side surrounds are being reinforced by the side walls/ceiling, whereas the back surrounds are all open behind them...
If I ever make a decision and get the 3808, then I can see how it treats my surrounds.
Having two rows of seating, I am unsure where to place the mic for 8 positions, but I'd figure that out to create that bubble of sound.