We provided some thoughts and leads, and you did an exemplary job of following up on them in a thorough manner, and taking them further. Truly a collaborative effort, on a difficult problem (as grounding and hum problems often are).
Regarding the popping issue, all I can suggest is what I said in my post dated 2-7-16, which raised the possibility that it might be resolved by a firmware update, if one is available. As I indicated in that post, someone had reported elsewhere a few years ago that what sounds like the same problem was **introduced** by a firmware update. Which suggests the possibility that the issue may have been addressed in a subsequent update. Aside from that possibility, I would suspect that the popping is being caused by a hardware problem, perhaps a faulty capacitor or relay, or a diode associated with a relay.
Enjoy! Best regards,
-- Al