I also thought 3803 is "not designed for M80" because the range on the receiver says 6-16 ohms. But it played well so I guess it is okay. But here is the official word from Denon:
"CAN I USE 4 OHM LOUDSPEAKERS WITH MY DENON RECEIVER OR POWER AMPLIFIER?
Yes, you certainly can.
To understand this a bit better, first realize that all amplifiers are designed to deliver a signal into an electrical "load" or resistance presented by the loudspeaker. We measure resistance in units called "ohms" (after the German physicist Georg Simon Ohm, 1787–1854).
Conventional wisdom makes an 8 ohm loudspeaker load the most acceptable because it "protects" the amplifier from delivering too much current. A 4 ohm loudspeaker can encourage a marginally designed amplifier to deliver more current than it comfortably can.
However, you should remember that a loudspeaker’s impedance rating is a nominal or average one: A speaker rated at 8 ohms may actually vary from 5 (sometimes even less) to 20 ohms or higher, depending on the frequency at which you measure the impedance. (Don’t worry about this too much -- good speaker engineers are well aware of these variations and take them into consideration when designing products.)"
