Official comment

1) Play a song and adjust the channel trim so the channel meter peaks at the first orange LED.
2) Press the channel cue button. Keep the channel fader down.
3) Ensure the Headphone Mix knob is turned to centre.
You should now hear the preview of that channel in your headphones (and adjust the volume; knob on the front of the RR).
If you press the cue button again (2), you should hear nothing in the headphones.
Press the MASTER CUE button (beside the red SYNC button), you should still hear nothing until you raise the channel fader.