If you want to play Redbook via Pure Music/DAC III, you could keep it simple and use one of the USB busses on the Macbook and the USB input on the PS Audio.
If you want to be able use all the features of Pure Music, like its ability to play back high resolution files (Like HD Tracks), my recommendation would be to relay on a bridge like the M2Tech HiFace, or the Halide in order to pass higher resolution files from the USB port on the Macbook, convert them to S/PIDF and use the coaxial input on the DAC, as long as you are planning on keeping the PS Audio DAC
If you want to be able use all the features of Pure Music, like its ability to play back high resolution files (Like HD Tracks), my recommendation would be to relay on a bridge like the M2Tech HiFace, or the Halide in order to pass higher resolution files from the USB port on the Macbook, convert them to S/PIDF and use the coaxial input on the DAC, as long as you are planning on keeping the PS Audio DAC