I set up two parallel audio paths in my mixer: one for broadcast/streaming, and one for the loudspeakers in the room. This way I can make extensive EQ cuts to the microphone groups feeding the P.A. without negatively affecting the broadcast mix.
I always use the main Left/Right output for broadcast/streaming/recording. Your main VU meter now displays the most important levels at all times, and your headphones are listening to this mix by default.
Matrix outputs are used for loudspeaker zones. The normal concert P.A. style is to feed loudspeaker matrixes from the Left/Right main mix, but I don’t do that. Instead, I feed the matrixes from subgroups, auxes or mixes only.
