If your Devialet product is equipped with it, you can use the PreOut output to connect headphones either directly - via a hybrid RCA-to-Jack cable - or through an external headphone-amplifier (for high voltage headphones for example).
Remember that you need to activate this output using the online configuration tool, assign a remote control key to turn the PreOut "On" or "Off" and define the correct output level depending on the type of headphones you use.
If you connect the headphones straight to the Devialet, leave the volume control field ticked on "Variable". If you plan to use an external headphone-amplifier then opt for the "Fixed" volume control.
It is also possible to connect headphones if you have a multi-amplifier configuration. In this case, you need to daisy chain the amplifiers using the DOUT on the master and the AES/EBU input on the slave in order to free the PreOut connectors on the slave for the headphones.