I would like to reiterate my observations here. If it is my money, I would go for a reasonble priced DAC with Optical In and use the Optical Out from Mac Mini.
I have no doubt about the sound output from USB of Mac Mini (or for that matter some other HTPC) but I question the stand alone DAC units. Including Rega Dac, the reasonably prices DACs are very good with Optical Ins but perform OK with USB in as the USB In is more "we don't want to be left out" kind of implementation.
I owned the CP-800 which has the best USB implementation that I did not use the Optical In any more but again the traditional manufacturers in HiFi industry (read as some British Companies) are purposefully slow to adapt to the new ways to connect and want to milk the USB slowly.
That is why I like the newer brands like Peachtree or Oppo which are new blood and are fine to offer more to the customer
Given today, I would stick with Optical Ins or Co-axial Digital Ins of the reasonably priced DACs!
My 2 Paisa..