My experience has been that almost any external DAC will be an improvement over the headphone output of a Mac.
One good place to start would be with a Musical Fidelity VDAC, either the original or version 2. I use the original VDAC in my office with an iMac and like it a lot.
One good place to start would be with a Musical Fidelity VDAC, either the original or version 2. I use the original VDAC in my office with an iMac and like it a lot.