If you're connecting the Macbook via USB to the McIntosh, then the McIntosh is decoding the audio data. What's happening is unfortunately it seems the Apple Music app isn't switching the sampling rate automatically. You can switch it manually using the 'Audio MIDI Setup' utility if you know what the actual sample rate is. Otherwise, macOS is resampling the music to match whatever is set in 'Audio MIDI Setup'. I don't know why Apple can't quite get their act together on this but it is what it is.
There are players out there that will switch the sampling rate automatically. For example, Kodi can do this when the 'Output configuration' setting in System -> Audio is set to 'Best Match'. However none of these players are integrated with Apple Music streaming.