I think as long as you don't adjust the volume on the back of the sub, and use the receiver trim levels your ok. If you've done the feature pack upgrade, make sure Dynamic Volume is not turned on for normal movie watching during the day, it will compress the overall dynamic range some. The Denon has a bit of a learning curve, even the GUI.
When you took your measurements, the seat backs were not in the way were they? Also, if you have leather, you should throw a towel or blanket over the top, as the seats can cause reflections and skew the results.