Tubes and their problems

Often popping sounds are just the result of dirty pins or poor contact of the pins with the tube sockets. When this occurs before you fret too much just try resetting the tubes in place or by switching channels. If that doesn't work you can try to retension the tube sockets.

Thanks, Newbee. What is the procedure for retensioning the tube sockets?

Microphonic tubes are usually ones that soon will blow.
You can tap amp chassis and if you hear tapping via speakers, your tube(s) are about to blow and you'll need to determine which ones by swapping between channels.
I would use sewing needle or similar that is able to go outside of contacting flaps and inside the socket and work on each one. Slight bending of tube terminals may also increase contact strength.
Tube that can't reach desirable bias may also be bad as well.

Clean the top surface of all tube sockets just in case. It is often arcing from one pin to another via dirty sockets that produces the pop or crackle.