Thanks Lilshawn, great advice, will do that next. One question: disconnect all speakers from the transformer just means remove purple black pink from E6, E1, E6, right?
Before I saw your message I did some additional trouble shooting along the same lines you suggested: I disconnected the wires between left tweeter and right speakers (the "solution" from a previous owner). Now only the left speakers should be working (other than the woofers)
Then I disconnected the pink and purple wires and then connected just the purple wire (with black still on E1): with purple on E6 on either left or right I get a good signal to my left speakers
I can't do the same with the pink wire, as the wires from the crossover to the right speakers (downstream of pink) are broken (I just hear the woofers when I do this)
But based on these results I would say the amp is good (left and right E6 have good signal) and left side of crossover is good.
Next step: restore the original connection downstream of pink to the 2 right speakers (the 2 wires that have been cut). If all is good, the reason for cutting the wires is a mystery but I am a happy man. If after restoring the connections a similar test with pink gives bad sound from left and right E6, I know the problem is in the crossover
Would you agree that it is safe to assume the amp is fine?