Just to get the ball rolling, if you wire multiple devices in series the load presented to the amp is additive ex, 2 8ohm speakers will be a 16ohm load to the amp. If wired in parallel 2 8ohm speakers will present a 4ohm load to the amp (remember speaker impedance varies over frequency but it is safe to use that 8 or 4 ohm number as an average).
You say the amp has 2 power settings or do you mean 2 power specs for 4 and 8 ohm's.
Give a little more specific info and I can help you more.
Makes model numbers etc.