Mathieu, welcome. Good that you decided to join the group. You correctly put "bi-amp" in quotes, since this is simply an advertising ploy of some manufacturers that isn't actually bi-amping and has no real benefit. The same amount of power from the one main power supply section of the receiver is simply funnelled into the speaker through two sets of output transistors(one previously unused for back surrounds)rather than one. No increase in available maximum power can result from this.
Your listening distance is a more significant factor as to power requirements than is room size. The room doesn't have to be "filled" throughout with the sound level you need at your listening position. The 1018 should certainly have no problem driving the M80s at the moderate level you mention unless your listening distance is unusually long.
