Hi Pete,
Thanks for flagging this up - I think this is a system that I’ve already been involved in.
I believe this is a unit that we have already had back to the US service centre for checking over and it has tested as absolutely fine.
We believe that the issue is caused by the combination of our output stages being DC coupled (we don’t pass the audio through a DC blocking capacitor and instead we use a DC servo circuit that actively manages the DC offset on the DAC output) and the rest of the system also being DC coupled end-to-end.
If the rest of the system is DC coupled all the way through to the outputs (speakers / headphones) then, as the muting relays operate, any DC offset (usually in the order of a few mV) will be amplified by the gain of the pre/power amps so there will be a click or pop as the muting relays open and close - as such the pop is ‘normal’ in that it’s a function of the system that it’s installed in.
Phil