The user guide isn’t really clear if this is possible, but have you tried to mute the LR output and then press the “listen” key on the L/R output. There is a good chance that this will allow you to listen to the LR output in the headphones, but not have any output going to your monitors. You also have independent control of the headphone volume, so you can adjust the volume of the headphones without changing the L/R fader position.