My system is similar to hgeitsman's - I always use AIFF with good results. The external HD is a 2 TB mirror by Cavlary connected to the Mac Pro with firewire. The iTunes music leaves the computer via a 1 foot long Toslink to a signal converter that, in turn sends the signal to the pre/pro via SPDIF digital cable. I tried USB out of the Mac but if you go to Utilities (Midi set-up) you will find that the USB "out" gives far worse specs than the "built-in Output" of the Toslink connection. The USB option gives you only one default setting of 44,100 hz 2 Ch 16 bit whereas the "built-in Output" (using the headphone jack with a Toslink adapter) gives you at least three options to choose from up to 96,000 hz 2 Ch 24 bit. For me the best sound results are at the 48,000 hz 2 Ch 24 bit choice.
iTunes and their AIFF format has been remarkably good both for importing and playback.
iTunes and their AIFF format has been remarkably good both for importing and playback.