Paul, exactly the same facts apply: this lower setting in no way "optimizes" the amplifier for handling the speakers with a lower impedance rating, as some imagine. This is purely a safety-related measure intended to limit the possibility of overheating, pursuant to UL requirements. It does this by limiting the voltage which is available on the power supply "rails", and following Ohm's Law, when the voltage is reduced so is the current and power. This of course reduces the possibility of overheating, but does so at the expense of maximum performance. Some lab tests(e.g., Sound&Vision)usually include the result of using the lower impedance setting and have found that it cuts the maximum power capability roughly in half.
