I've received a number of questions about using a Mac as the basis for a music server and I thought I would start a thread dedicated to the topic. My plan is also to prepare a podcast or video that explains the world of HD and Macs. I'll try to get to this weekend.
In the meantime, here's the short version.
The Mac makes a great music server for 2-channel HD content, but is not capable of playing multichannel HD files. It's fairly straightforward to download 96 kHz/24-bit 2-channel stereo files, place them in your iTunes library and then set up your Audio/MIDI Setup application to handle 96 kHz/24-bit files to the DIGITAL OUT of your system.
If you have additional hardware such as a Benchmark DAC-1, you can also direct the USB output to that device by clicking at the bottom on the "+" sign and configuring a new device.
You may have to restart iTunes for the changes to take effect...to get the 96/24 HD standard working.