Thank you for keeping us up to date. I was thinking along the lines of a small DC spike when making input/volume changes. This would be noticeable with amps that do not block incoming DC.
With the buzzing, the culprit could be one (or both) of the new cryo treated tubes. Can your dealer re-solder in new tubes to try that out?
With the buzzing, the culprit could be one (or both) of the new cryo treated tubes. Can your dealer re-solder in new tubes to try that out?