I currently used Spotify Connect with my Yamaha receiver. Works great! BUT, if I have played Spotify in its last use connected to the receiver, it uses the reciever not my phone to play music.
Example: Earlier in they day listened to Spotify on my reciever at home. Later that day I am in the car connected to Bluetooth. I hit play on the first song of the play list and nothing comes out. At home, my husband is watching TV and the reciever switches over to Spotify and starts playing! Not much fun when the TV sound needs to be up higher than Spotify!
Have this happen enough to know it seems to be the expected behavior. If the recieiver is not on, it will turn it on and start to play. I will have to be very careful on morning commutes as I leave before my husband gets up!
Seems resonable that at least by default it should play first on the device you have hit play on, not the last connected device.