Just a hint regarding Pi housing. I used Apple TV 4G box for my Arduino project (weather station). It also could be useful for audio projects powered by Pi. All Apple boxes are nice and solid so it's possible to make an 2-unit streamer. One box for the Pi and HAT and another for linear PSU as well.
The major question is the PSU temperature, but there is a plenty of space to make a big hole in the side wall and install the heatsink.