Mike, what NAD is saying is that their amp has a automatic circuit to accomplish what some others do with either a manual 4ohm switch or menu setting. In either case the rail voltage available is reduced, which in turn reduces both the maximum current(I=E/R)and therefore power(P=ExI, or P=I(squared)xR)available at four ohms. That's why their amp isn't rated for the higher wattage that is normally available at 4ohms if the full voltage is still supplied at that impedance.
Despite that, it's highly likely that the NAD would be more than adequate at 4ohms. I'll repeat my previous suggestion that if your 7200 is plenty for for the M80s, so would the similar channels of the 3480 be for stereo M80s.
