The "Monitor Edit" menue (touch screen) allows you to select any mixes directly. Press the Edit button in the "Monitors" section of the console, the menue will show up. Select Phones as output. Press "Main" to listen to Main L/R and "Mix 1" (example) to listen to this mix. If Mix 1 is a stereo subgroup, you will listen to this stereo subgroup now. Press "Main" and you're listenting to the Main mix again.