The subs can be used independently if you have the channels panned hard left or right. A mono mix to the house can be sent from your main outputs and you can assign each of the four sub groups to a separate slider. A good reason to use this feature is if you have several singers and want a single slider to control the volume without changing the rest of the mix. You could have:
Group 1 - singers
Group 2 - drums
Group 3 - instruments
Group 4 - backing trax
You can also use the sub outs to send a monitor feed to a powered speaker ... say, for example, you play with sequences or trax - you could send the drummer a feed with only the necessary parts from the sub group output.
Just a suggestion of one way to use the sub groups. Enjoy!
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