Sub stands for subgroup, not subwoofer. You can assign certain channels to feed subgroups (1-2/3-4) rather than your mains, assign the subgroup to the main (LR) mix, and then control them all these channels with one fader! This is very useful for controlling a large blend of inputs, such as a drum kit or backing vocal mix.
Alternatively, you can assign channels to subgroups for use as extra stereo outputs with the sub outs. Get creative with your routing!
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