Hey Dekalboy,
In order to listen to the input AB you dont need a THRU machine…
Go to the Mixer page, at the right side you will see GAIN AB and at its left “DIR” … this conctrol the “Direct” signal that comes from the AB (or CD inputs) and pass them trough the master output… but you wont be able to affect them in any way… just rout them out.
Hope it helps! 
Cheers