Word Clock connection X50D

Hello everyone this is my first post so be gentle with me. I am looking for more information and practical use of the Word Clock connection on the X50D. I use an Antelope Gold DAC which has a word clock input. This works if connected to the X50D word clock connection but only if coax SPDIF or AES PCM modes. There is an enhancement of the sound quality - stereo image and timing are improved. When the X50D is connected to the DAC via USB the clock connection does not work with no transmission of signal and no sound. Anyone had any experiences to share please?