PT, yes you're answering your own question to a considerable extent by noting that you have no problems now with 4 ohm rated speakers. You should also note that the M80s have slightly higher sensitivity than average and so require less power than many other speakers, and that over a majority of their frequency range they could be rated at 8 ohms.
As to a setting on the receiver for lower impedance, as was said, this should never be used. All it does is limit the voltage(not current)which the power supply section can output with a resulting reduction in current and power capability(Ohm's Law). This helps satisfy UL requirements to help prevent overheating, but tests have shown that it cuts maximum power capability roughly in half.
