I would say there is no definitive answer, you could do some measurements to find out what the best/flatest setting would be be for your room, but it still comes down to personal preference.
I, like Jason, run my M80s at various settings just for the fun of it, sometimes it sounds great other times not so much. Although I wouldn't take the M60s down to the 40hz level as you have a sub there is no need; also I believe their specs are only slightly below 60hz. I wouldn't hesitate to try them at that 60hz point and see if you like that setting better. If you don't feel there is much difference, then keep them at 80 and take a little pressure of the Onkyo.
