The 3800 can certainly drive 4ohm speakers, including the M80s, to a very high sound level, but that suggested setting in the manual should never be used. All that a lower impedance switch or menu setting does is to reduce the maximum voltage that the receiver can supply, and following Ohm's Law, when the maximum voltage is reduced so is the maximum current and power capability of the unit. This is a safety related feature required by UL overheating testing, but can cut the maximum power capability roughly in half. Disregard the suggestion in the manual.
