I think I got to the bottom of this mystery. Tightening up the socket jacks helped. Using thicker gauge wire on the filaments helped.
There remains a slightly diminished output in the right channel. I suspect a cold solder joint - somewhere.
Since I had not removed the power tubes for 2 years, I do not know how the sockets jacks lost tension (or contact). Heat? Vibration? The sockets jacks and the pins of the tubes are not at all corroded.