Tube amp crackle noise with pre-amp volume at zero.

Most likely noisy small tubes, inherently noisy. Could be power tubes but usually this is not the case. FWIW the volume control usually appears in the circuit before the tube amplification and only attenuates the signal from the source. Next tubes you get should be 'low noise' tubes especially if you are buying NOS tubes which are costly.

You might just try shifting the small tubes - left channel to right channel, etc. Some times the issue is not the tubes so much as the pin contact in the sockets.

Try cleaning the pins with denatured alcohol or 99% isopropyl and carefully clean sockets with a non bristle pipe cleaner and a tiny amount of alcohol.... then when everything sounds great , sitting down with some of your favorite alcohol is in order.

These weren't the 12AU7s, but the power tubes.  GE EL 34 Fat Boys.

billtro -I also own a PrimaLuna (DiaLogue Premium Integrated). This amp has the "Bad Tube Indicator* feature that puts the unit into protective mode when a power tube goes bad and tells you which tube is the culprit.

This feature has proven its worth twice.  The first time was a little over a year and a half ago when the #1 KT88 failed. The second time was a couple of days ago when the amp shut down and the BTI showed a bad KT88 in the #4 slot.  

I did notice some sporadic crackling/small pops before each failure.  So, if your PL has this feature, you can rest a bit easier knowing that the BTI circuit will kick in a shut down the amp before any damage is done.

I've ordered a new set of Gold Lion KT88's which should arrive on Monday.  

yes bti red led will light up on the power tubes if one operates outside of what the primaluna amp bias range allows

coughing tubes usually small signal tubes... try fresh set, or move tubes side to side to isolate

primaluna supplies sino shuguang tubes labelled primaluna... primaluna gets them with more quality control but the qc is far from perfect, and the modern tubes often wear out get noisy faster than old stock ones due to poor materials poor manufacturing of the tubes