I had issues using a short (3 ft) microphone cable between my HD500X and an L2t. A low level random clicking could be heard. This went away when I switched to an AES / EBU cable. I expect that the drivers and receivers are terminated for a 110 ohm cable, and the lower impedance of the microphone cable was causing occasional bit errors in the sample stream due to signal reflections.
This would probably be much more evident if the speaker was just a dumb D/A, but I hypothesize the DSP in the amp filters down the bad samples to some extent.
I highly recommend using 110 ohm AES /EUB cables for digital - there really is is a difference.
