Thanks. I suppose it could the the power line. I'd have to try another location to verify. I was told early production units had a loose ground, and wondered whether that could impact the volume. A UK forum contributor found the same issue, calling it a clicking, rather than a popping, sound (but I have no transformer hum, as he reports).
http://www.harbeth.co.uk/usergroup/showthread.php?41-Harbeth-SHL5-specific/page13
The Krell integrated was noisy in operation, it seemed to use the same numeric volume control as my Arcam pre, but with an audible click with every change and it hissed through the speakers worse than a Naim pre, plus there was a very noticeable mechanical hum
http://www.harbeth.co.uk/usergroup/showthread.php?41-Harbeth-SHL5-specific/page13
The Krell integrated was noisy in operation, it seemed to use the same numeric volume control as my Arcam pre, but with an audible click with every change and it hissed through the speakers worse than a Naim pre, plus there was a very noticeable mechanical hum