M, numerous times here and elsewhere it's been pointed out that the "feature" involving a lower ohm setting is worse than useless if performance is the criterion. This is simply a safety related setting intended to help prevent overheating. If the units in question don't pass the UL temperature tests when driving a 4ohm load, the setting is required for liability reasons. It functions by reducing the maximum voltage that the power supply section of the unit can output and, because of Ohm's Law, when the maximum voltage is reduced so is the maximum current and power. So, although it reduces the chance of overheating, it likewise reduces the maximum performance possible. Lab tests have indicated that the maximum power is reduced on the order of 1/3 to 1/2.
Regardless of the ohm rating of the speakers used, the setting should remain at 8ohms. All amplifiers are 4ohm capable, the question is how capable. It shouldn't be assumed that the particular source material used in a particular setup at a particular volume level won't be handled well.
