Hi Louis,
Yes, actually, the least expensive Denons and Sherwood Newcastles will drive the 4-ohm M80s within reasonable limits in terms of rated output power and room size.
You should know that when you set your Onkyo to the 4-ohm setting, you are actuating a current-limiting circuit that greatly lowers the voltage and current flow delivered to the output devices, thus limiting your Onkyo's power output to a fraction of its rated power.
That's why when you continue to increase the level, you risk clipping the amplifier or pushing it into excessive distortion because the voltage rails are "limited" by the 4-ohm setting.
