Here is the update. I put in a new Pass Labs amplifier and still no sound, so I though it was SL. Then as I was walking by SL, I accidentally hit the speaker cable and sound came on from both speakers. What I think happened is this. Previously, I mistakenly connected the red terminal to -, rather than +.So I reversed it to correct this error and, in retrospect, problems started after that. I didn’t suspect this because they were on pretty tight. But after the switch, the spade-terminal contact must not have been very good-probably the angle of it was a bit off when I reversed them. Roger West thought it was the safe mechanism on the amp. I feel like an idiot, but it is a warning for those of us who now use these stiff python-like speaker cables. You think they are on tight because the bolt feels tight but you are not getting good contact.