Originally Posted by gregcss
When I was setting up my SR6012 I did some searching on that setting since my speakers are 4ohm (design) 6ohm (nominal). The consensus was to leave any/most AVRs at the default setting of 8ohm. This is not specific to just the Marantz. I've had no issues leaving it at 8ohm.
I'd imagine that putting in 4ohm mode increases distortion and heat while the power output is increased.
The 4 ohm setting is intended to reduce the heat generated by the unit, not increase the maximum power output level, which in fact goes down.
The 4 ohm setting causes lower voltage taps to be used from the power transformer. This is controlled by a relay. This in turn reduces the rail voltage across the power transistors. This lower voltage reduces the heat generated by the power transistors. So far, so good.
The lower rail voltages also dramatically reduces the maximum power available from the amplifier. The amplifier will clip at lower output voltages. Not so good... The amount of energy stored in the capacitors in the power supply is also dramatically reduced because that energy is proportional to the square of the rail voltage.
A good reason to use the lower impedance setting might be if the unit were used for background music in say a doctor's office or in a house with speakers spread throughout the house for background music. In these cases not much power is required so a cooler running unit, especially if the unit is in say an enclosed closed space, would be beneficial. The low impedance setting would also reduce the the electric bill, which could be important for units run much of the day.
For normal use of the unit it makes sense to use the 8 ohm setting since a higher maximum power output is available.
It might seem that the lower impedance setting would protect the AVR from overheating and damage when driven hard. This is true, but this constant protection is not actually required. While the details vary, AVR's typically have sophisticated protection systems that monitor, for example; total output current of the unit, and temperatures at various locations such as the heat sinks and around the power transistors. These systems turn on fans if available, or increase the fan speed if possible. They also change the output rail voltages just as the occurs with the 4 ohms output setting. This occurs at high outputs however when the lower rail voltage setting is actually protecting the amplifier sections from damage. The protection system will ultimately shutdown the unit if it finds the unit is in danger of damage. This protection system reinforces the recommendation that the impedance switch be set to 8 ohms and let the system protect from over current or over heating conditions.