Okay, an update:

Just before pulling the board from the chassis to revisit the output transformer wiring, my son says: why don’t you try unplugging one speaker or switching where they’re plugged in?

Silly idea, won’t do anything, but it works …

One speaker plug seems to be the problem. Seems to be shorting out. If I open it up, then it works, no problems. If screwed together tightly and the wire or spring stress relief is wiggled, then sound for both speakers becomes intermittent.

To those more experienced than I am: does it make sense that shorted wiring in one speaker connection could stop sound from both?

Please, educate me.