Many thanks. That solved the issue. At the same time, I also found that I could move Cue Sends to Pre-Fader so that the headphone mixes remain audible with the Stereo Out muted.
Thumbs-up given 
Many thanks. That solved the issue. At the same time, I also found that I could move Cue Sends to Pre-Fader so that the headphone mixes remain audible with the Stereo Out muted.
Thumbs-up given