Ian, that's correct that connecting two speakers of equal impedance in parallel has the effect of reducing the net impedance to one-half the previous value. As the Audioholics article explained, setting the receiver to a lower impedance value is simply a safety type of feature required by various regulations to help prevent overheating; it in no way optimizes the receiver for the lower impedance. By reducing the maximum voltage that the receiver power supply can deliver, the maximum current and power are likewise reduced, following Ohm's Law. So, although the chance of overheating is reduced, so is the maximum performance possible. Therefore directions in manuals to do this should be disregarded and the impedance setting left at the higher value.
