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).
Depends on your DAC. If it has a good USB input, that would be the way to go. If not, USB to spdif converter into your DAC would be next best probably, followed by min-toslink into DAC.
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
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