You got it, just like everything with the human body it adjusts to its environment. Just like after awhile at a rock concert the SPL is no longer a consideration, then you leave and the ringing in your ears is now noticeable, you are experiencing the same effect.
I also agree with everyone else, to noticeably louder than the Denon is capable of, you have to be looking at twice the power to start with, ~300W @8ohm.