Try just running the m2's as fronts with the speaker wire and see if you get any problems. If no problems, switch to m60's as fronts (using same wires) and see if you get a problem.
Also, you list m2's as "rear" speakers but if you are running a 5.1 system the surround speakers are actually "side" surrounds and should be placed to the side or slightly behind the listening postion.
Edit: just realized you were only getting the error in 6 channel stereo mode. My guess is that the receiver is being over driven as it is trying to play to all speakers at the same time. if you stick with 2 channel stereo and 5.1 surround for movies do you get the same error?
5 channel stereo mode is pretty taxing to the receiver at loud volumes, at least it makes my denon much hotter than 5.1 movies or 2 channel stereo does.
