Audible illusions Mod 3a right channel popping

If you haven't fixed this yet, check the pcb traces that connect the Elma atten,s to see if all the copper pcb traces are soldered down. I seen one that had a floating broken copper trace that would bounce every time you turned the knob and the clunk would make make contact causing the pop.
Think of a ring around your finger that is too big. That "ring" is what was around one leg of the Elma switch and the vibration would contact the adjacent pin making the noise.
Simple fix in this case but hard to see at first

Regards

If B+ voltage somehow got into the signal path(very unlikely even with blown tube) it may've damaged the coupling cap. Other than that I don't see the reason why coupling cap is the culprit.

I vote for changing the electrolytic caps, they are not expensive, and all easy to change except one; leave it.

Yes you may have D.C. voltage sitting on the volume control in which case a coupling cap may have failed. Did this problem exist before you had the tube sockets replaced???

Is right channel popping when you touch the volume control or regardless?
Could also be DC caps that need replacement.