Thanks to all. To oldhvymec4: The front speakers are indeed set to Large in the speaker set up of the Denon AVR-2807. I thus understand the crossover setting on the Denon is irrelevant and the low pass filter setting in the M&K will control. On the other hand audiorusty appears to say the low pass filter on the M&K is irrelevant-- yet it is suggested I set the low pass on the M&K to its highest available, which in this case is 100Hz. These seem contradictory.
Eric_squires: The 49Hz figure cited is the manufacturers spec for the 805S. I haven't any instrumentation to determine precisely. As indicated that figure is ±3dB and the M&K rating is ±6dB so there is a fractional difference where the blended frequency should be which I also cannot calculate and thus have used the speakers 49Hz as a "compromize".