By default, each of your M-48's will hear all the sources that are assigned in your M-400 REAC B Output Patchbay, or all the sources from your standalone Digital Snake stage unit. If you want to mute any of these sources so they are not heard in the M-48, go to the SOURCE LEVEL/PAN menu of the M-48(s) and set the unwanted source's fader level to minimum. This will silence that source in that M-48. Repeat with additional sources or additional M-48's as necessary.