What is the factory default value for the master output’s attenuator level (-6/-3/0 dB)? When do I need to change the value?

The factory default value is 0 dB.

If sound distortion does not improve after the [MASTER LEVEL] and [TRIM] controls are adjusted, the maximum allowable input of connected power amplifier and speakers might have been exceeded. In such a case, use the attenuator level setting to set the master output to a suitable level. The output level can be reduced to 70% at -3 dB and to 50% at -6 dB.

This setting is not linked to the master level indicator. For instructions on changing the setting, see “Changing the settings” in the operating instructions.