I think the headphone out is also an optical out on the Macbook Pro. If that's correct and your DAC has an optical input, then all you need is a mini-toslink to toslink cable (or an adapter and a toslink cable).
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I use the optical output as explained above with better results than USB. I believe you also have a firewire connection? but you would need an interface to make the connection if your DAC does not have a firewire port. Firewire is best used as a direct connection say to a weiss dac. It is far superior than the other two options
A low-jitter USB converter will easily beat the Toslink output from the Mac. The best way to go is I2S, but your DAC must have this input. Next to that, S/PDIF coax will be great. Whatever cable you end-up with, it should be a high quality cable. Makes a big difference. Cables in the $100-300 range will not get you there.
Firewire is no better than USB. It all depends on the implementation/design. Either can be great.