What music sources can be sent to more than one A1-MR simultaneously?

Sending music to multiple A1-MRs can be done with any source after grouping the systems together. Start by playing music on one through the app, Airplay, or Spotify Connect. Then in the app, drag any additional systems onto the system that is playing. They will synchronize and you will be able to control the volume of every system separately. The app will remember these settings until they are changed.