Microphonic from Vacuum Tubes?

Does the loud pop occur in just one channel or both channels, and if in both, does this happen simultaneously in both channels?  Does it happen when the music is playing at high volume levels or under other consistent conditions, or is it seemingly random? I am not a technical expert, but, this sort of additional information might help others diagnose your problem.  I have not heard of large occasional pops being an issue related to a tube being microphonic. Can you reproduce that noise by lightly tapping on any of the tubes?  If a tap gets you the same kind of pop, that tube might be microphonic.  Otherwise, I would suspect that something in the power supply, maybe a capacitor, might be at fault or perhaps you have a tube socket going bad.   I have tubes that occasionally, quite suddenly, develop loud crackling noises that last for a few moments and up to a few minutes and then the noise goes away (I was told that this is caused by something flaking off in the tube and falling on the heater, with the noise lasting until the flake is burned away).  The loudest pop I've heard involved an output transformerless amp where a tube socket was going bad--that pop was loud enough to almost trigger a heart attack.   

I doubt that tube dampers will do anything for the cause of the popping noise.  This is a problem with a tube, a problem with a tube socket (this is the most common source of such problems) or a problem somewhere else where a charge builds up and then suddenly discharges (probably somewhere in the power supply circuit). 

Tube dampers work to minimize microphonic ringing, but that is not an issue here.  Do they improve the sound?  In some cases they do, in others they make the sound "drier" which may actually make the sound less pleasant.  Consider tube dampers and other damping products (platforms for gear, weights to put on the chassis, etc.) as tuning devices--they alter the sound which means they can also alter the sound in the wrong way.

If you suspect capacitors in the power supply, you can at least do a visual inspection.  Bad caps often leak when they go bad, or even more commonly, they bulge.  The bulge is most evident at the ends of the capacitor, particularly the top.  Compare the capacitors to each other and if one has a top that looks like it is sticking up, it may be the bad one.