Peter, there should be no audible difference between the amplification provided by the 2807 and 818 within their designed limits(with the 818 having just a very small, probably insignificant maximum output capability edge).There should be nothing in the way of greater treble output that would require a break-in of either the speakers or your ears.
You should follow the suggestion CV mentioned, i.e., running the speakers on the 818 alone to see if some problem has developed with the XPA-5. If the problem still exists(with tone controls and Audyssey correction not in operation)there's a fairly remote chance that the particular unit of the 818 which you got is defective.
There shouldn't be any difference in DVD vs Blu-ray audio which would cause what you report.
