See this post for what I have already done to this amp:
http://www.audiokarma.org/forums/showthread.php?t=181257

Tonight while enjoying some mono Jazz, it suddenly made a loud "pop" sound which the music immediately dropped to a very low and distorted volume. This is getting frustrating! Any ideas?

bump

Check all the obvious stuff first: reseat tubes in their sockets; clean the pins if you haven't (focus on preamp/driver tubes and phase splitters); check the interconnects; check all switches and selectors... I find little 'issues' all the time that seem, at first blush, to be much worse problems.

Otherwise, big pops are often associated with capacitors.

I cleaned all of the tube pins and sockets, switches and controls once the new tubes arrived. I am leaning toward a capacitor issue as some of the ones in the preamp section were very hard to get to, so I have not replaced them yet. i will have to buckle down and change those, and see what happens...