Once you set the receiver to sub = no, the mains automatically are set to large and no crossover for them, the rest of the speakers I set at 80hz and see how you like it, then move up or down 10hz and see which way you like it better.
As 2x6 said, once you get the sub then set the M80s to 60 or 80hz depending on your tastes and small setting. I would also still try them set to large and the Sub set for LFE+mains with the crossover still set at 60.
Not sure why with sub =yes, even though you have no sub, would give the 80s a better low end, as you would be cutting off the lower frequencies to them. Could be the Denon isn't powering the M80s well enough the handle the really low stuff.
