MT, this has been discussed numerous times, including another thread here now, but again, despite the language in the manual, never set the receiver to the lower impedance setting(the higher one is always the default), regardless of whether it's termed 4 ohms or 6 ohms. This is a type of precaution required by the UL to help protect against overheating. It in no way "optimizes" the receiver for a lower impedance speaker, as some imagine, but in fact degrades maximum performance capability. It does this by reducing the voltage that the power supply section of the amplifier can output, and because of Ohm's Law when voltage is reduced so is current and power. The maximum power capacity is cut roughly in half, tests have shown, so although the potential for overheating is reduced, so is the potential for supplying power free from audible distortion or even clipping.
