Am I missing something? Does Google Music determine the quality based on the speed of the internet connection? I was tethering from my Nexus 5 over Bluetooth and found my speeds were pretty dismal (2 Mbps down when my T-Mobile LTE usually gives me around 30 Mbps).
Spotify lets me select the music quality in tablet mode, but I ditched them when they weren't handling notifications properly by ducking (they used to pause). They finally fixed the notifications, but they don't support the Chromecast, so I'm keeping Google Music until that happens.