What is this audio chain we keep talking about?
All equipment located at your station (computer, K2500, etc...) sends audio signals to the Mackie mixer at your station.
From your mixer, audio signal can be sent to your headphones or the rack.
Stations 1-7 go into the top mixer (mixer A), Stations 8-10 go into the bottom mixer (mixer B).
Mixer B sends signal to channels 15&16 on Mixer A. The signal then travels out the subs on Mixer A and out to the location you have specified (defaults to the room's speakers).
Below is a diagram showing the audio chain:

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