PS Audio Digital Link III - Best Connection to MBP

I use a Toslink cable, (Toslink to mini stereo) to connect my Macbook Pro to my DL III with good results. The Toslink and Coax on the DL III will accept 96/24 files whereas the USB only acepts 44/16 (standard Redbook Audio).

Invest in a decent Toslink cable. I'm using a Wireworld Super Nova 6 which sounds decent (the Toslink connector is all plastic and I wish that it was a little better made and more durable).

Also, have you tried 'Decibel' FLAC player for the Mac? I liked it better than Pure Music and it is a good bit cheaper. However, I am playing mostly 96/24 files downloaded from HDTracks.com. I don't interface or play files from iTunes generally.

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

I have the PS Audio DL III and I do not recommend using the USB input without placing a USB/SPDIF reclocker in the path before the DAC. The DL III's USB implementation does not produce very good SQ compared to the toslink/coax inputs. The difference was easily heard in my cheapo system.
In fact, the USB input of the DL III is limited to 16/48, so if you want to play 24/96 or 24/192, you must use the DL III's toslink or coax inputs.
All that being said, the same USB/SPDIF re-clocker can also improve the SQ of your toslink and coax outputs from the Mac, if the re-clocker has those inputs.

of the three inputs on the Dlink III I find the coax input with silver stranded digital cable to perform the best. Using digital OFHC copper coax was not that different from the toslink quality with glass optical cable. Realremo is on target about the USB input with PC... don't know if same limitation applies to MAC