One other option:
- Connect MacBook Pro to receiver via HDMI.
- Use Plex Media Player on MacBook Pro to play music/movies/etc.
- Use Plex app on iOS to control PMP on MacBook Pro (see Plex Companion).
For music, audio will be converted to PCM. However, since PCM is lossless, there should be negligible quality loss for the FLAC files. I believe the PCM will be at 48kHz/16bit, so any hi-res files (96/24, etc) will be downsampled.
You’ll need to configure PMP to send audio to the receiver, not the Macbook speakers. See this post for help with that (the post is about 7.1 audio for movies, but will work for music as well).