Thanks for your thoughtful reply. I too have not found settings in either Music or Bluetooth to account for what happens, which is why I ended up here. I can't say whether my receiver has a "resume play" command (I've scoured the manual), but I don't think so for the following reason. Situation: Music is not open. SonicWeb is not open. Turn on receiver and select Bluetooth from the receiver control panel. Music (only) launches; SonicWeb does not open. Quit Music quickly and open SonicWeb; play streams (Music does not re-open, all is well). Quit SonicWeb. Turn off receiver. Later, repeat the same process. Music again launches, etc.
If the receiver was sending a resume play command, why doesn't SonicWeb open? It was the last application to be playing. Or is only Music able to receive a resume play command via Bluetooth.
Also, SonicWeb doesn't use a separate set of media buttons. It has its own Play/Pause/Stop/Record buttons (no Next or Previous) within its GUI. SonicWeb doesn't use a web browser interface, despite the name of the application. It's free-standing.
I suppose my best option at the moment is to contact the receiver manufacturer (Yamaha) and inquire about an undocumented "resume play" action.
Parenthetically, I had the same issue with iTunes on an older Mac. But, as I mentioned, I found some Terminal commands (at StackExchange, I think) to prevent activation of the RCD (remote control daemon). Does the same autoplay behavior with iTunes and Music imply that my receiver is at fault?
Many thanks.