I was just knocking around with the my Nad 3020i today, and was trying to use the impedance selector at the back to change it from 8ohm to 4ohm but found that the switch is locked on, it seems it needs to be unscrewed or something to change it. Any idea on how to do this?

I'm trying to understand the meaning of impedance matching, still trying to figure it out. For example, my mission speakers are at a nominal impedance of 8 ohms, so since the Nad has a 4ohm option, wouldn't this give more power to the speakers?