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Spotify Connect uses the highest quality available when streaming to receivers.
If you select normal quality on your phone and then use the app to control Spotify on your receiver, the receiver will still play at the highest quality which is 320kbps.
This is because your receiver is streaming Spotify directly from the servers, and not from your phone.
You are only using your phone as the remote to control it.
Update: I would still recommend changing the streaming quality to the highest in the settings because it may change/differ between platforms.