The MAC mini has an optical output that is fantastic and plugged into a good DAC is all you need.
Personally, I would not allow the S/PDIF conversion to be done inside a computer. If you use the Toslink on the MacMini or their towers or laptops then the conversion is being done in noisy electronic environment of the computer. I would bet you'd get an audible improvement by taking that conversion outside of the computer, as it would be with a USB DAC, USB Converter, or better still, a device like Empirical offers that goes direct to I2S which is the native language of the DAC chip.
Marco