In the owners manual it says:
External amplifier: when an external amplifier is connected, sound quality can be improved by stopping the power supply of the built-in amplifier. Power-ic off, use this mode when the front and rear amplifier of the unit are used to drive an external amplifier connected to the speakers. in this setting, the head unit's internal amplifier has no output to drive speakers.
How do I get to the power-ic off setting? It just tells what it is, not how to turn it on/off.