Hi,as the V6 tube(2nd&3rd/gainStage-CleanGreen) is not at fault
,another option is V1 tube(1st/gainStage-allChannels).
One 'half' of V1 handles the Green,the other 'half' for Blue&Red.
Yes, you'd expect ALL channels to have the popping;even more on Blue&Red,
but it is worth checking before shipping.
Also,while the popping is happening, when the volume is turned up&down the noise
will vary accordingly?
If not (i.e. it stays constant in loudness), it's something further along the signal-path;
PowerTubes, or something really random like the SpeakerCable/connections with intermittent contacts
(apologies if you've already been through this stuff already)
....easier to change than internal components & connections ,which unfortunately would be likely.
One other 'easy' thing to check is the channel-switcher on the Back of the Amp;
flick it ,and the other ones,a few times (Low Volume!/footswitch NOT plugged in).
It'll confirm they're properly 'latched'.
Kind Regards,David.