tomtuttle, you are a gentleman and a scholar. Of course you were right about a crossed wire. I should have known since the speakers were disconnected for interior painting. Once of the connections hadn't been redone properly and both wires were touching the same terminal. The Sherwood Newcastle works great now. I was cursing it, but actually it saved itself. Thanks!

I'm still interested in advice about iOS-controlled receivers, as that's the direction I'd like to go, though with much less urgency now.