Asap, if you want to follow the owner's manual literally, you should use option 2. However, I wouldn't do this. When I have a 8/4 ohm amplifier switch and 4 ohm speakers, the switch stays at 8 ohms. The 4 ohm setting restricts the amount of current flow at high levels and possibly would affect the sound occasionally. It's highly unlikely that your 595 would do anything but run a bit warmer with the speakers you describe.
Since you previously said that you'll be using a sub, the steep rolloff of the M2is below 70-80hz shown in the NRC measurements should be acceptable. You would set them as "small" on your 595 and the sub would then be given primary responsibility for the low and mid-bass(don't forget to bypass the sub crossover if it has that option or turn it up to the maximum frequency to get it out of the way of the 595 crossover). The quality of sound should be nearly identical overall compared to using the M22s.
