First, I would recommend to power the mini from an external Linear supply, preferably a Hynesdesign.com SR5 or SR7. This will make a huge difference in dynamics.
Second. I would recommend to use Amarra symphony for player.
Third, get the "audio optimization guide" from iTunes iBooks and do these tweaks to the Mini.
Third, replace the HDD with a SSD for the OS to run from. See for details:
http://www.empiricalaudio.com/computer-audio/recommended-systems
Fourth, try all of the USB ports on the Mini to find the best one. I know which is the best on 2009 Mini.
Fifth, make sure you are ripping CDs using XLD with Accurate Rip enabled. Much better than iTunes:
http://www.empiricalaudio.com/computer-audio/
Finally, if you are in the $5K price-range for a DAC, consider the Overdrive SE, just reviewed by audiostream.com. It came out on top when compared to other DACs, like Bricasti and Analog DAC etc.. You will need dCS Vivaldi to come close IME. There were Vivaldis in rooms on either side of my room at the last RMAF and I'll take the Overdrive:
http://www.audiostream.com/content/empirical-audio-overdrive-se-usb-dacpre
Steve N.
Empirical Audio