Bob, typically company representatives, primarily for legal liability reasons, won't recommend using equipment not specifically UL listed at 4 ohms with 4 ohm speakers. Nevertheless, Yamaha and other similar receivers have often been used successfully in that way with M80s and other 4 ohm speakers. Also, as Doug mentioned, the M80s are a relatively easy 4 ohm load because of slightly better than average sensitivity and a relatively flat impedance curve.

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Enjoy the music, not the equipment.