Dave, your friend is correct at 3.4 ohms if the two parallel impedances are 6 and 8 ohms; David is correct at 2.67 ohms if the two are 4 and 8 ohms. The calculation is that the sum of the reciprocals of the individual impedances is equal to the reciprocal of the combined impedance; e.g., 1/4+1/8=1/X, 3/8=1/X, X=8/3, or 2.67 ohms. That's the math, but we really don't know what the actual real world result will be at any instant, since that depends on what the impedances of the two speakers would be at the frequency being played at that instant and we have no M60 impedance curve available, apparently.
Having said all that, don't worry about it. The 1909 doesn't know what the impedance of the speakers it's connected to is, just that it has to supply a certain voltage and current at each instant. If the short term current was extremely high because of an extremely low impedance(not the numbers we're talking about)the protective circuit would shut down immediately. If over a considerable time(which you wouldn't likely allow)the current caused overheating to build up, again the protective circuits would cause shut down without permanent damage.
Enjoy the music, not the equipment.