Bruce - your current arrangement is less than optimal, IMO. You should use an external USB/SPDIF convertor. This would add a standard USB cable to the equation, as well as a digital cable of your preference (Toslink or Coaxial) depending on the device. The latter would go to the digital input on your MF. Empirical Audio's Offramp would serve that purpose. I use a Waveterminal U24 for the same purpose, but those are no longer avaialable, or I would recommend them. I believe Hagtech offers an alternative, as do several other companies, both in pro-audio and specialty audiophile. They are available at various pricepoints, as most things in this hobby. You can get a convertor from PartsExpress for $20, or spend $1K+ on one from Empirical or spend several thousand on a new USB DAC from Wavelength. Since the conversion and clocking is critical I'd say it's an investment you shouldn't take lightly (skip the Parts Express route). The idea is take the conversion of your computer audio files output, into SPDIF, outside of the electro-magnetically noisy environment of your MacBook (where it currently is taking place given the arrangement you describe). USB seems to be the best interface for doing that. I'm no expert, consult someone like Steve from Empirical, or Gordon Rankin from Wavelength for various points of view on the subject, complete with technical details up the Wazoo. There are also plenty of threads in the archives here and over at PCAudioAsylum that may give you additional real-world opinions and advice. I think most would agree that getting the coversion/re-clocking out of the noisy computer environment will be a significant improvement.
Marco