I have been using an iMac as a source with Pure Music and a HiFace USB to RCA adapter, and I find the sound to be much better than the optical output from the iMac using iTunes which is converted to digital coax.
This isn't a very expensive upgrade. Pure Music is 129.00, and the HiFace adapter is 150.00. Either Pure Music or the HiFace adapter, without the other, is an improvement. Pure Music is limited to the Mac OS, but the HiFace adapter is both Mac and PC compatible.
There is a lot of information on the HiRez circle, including an inexpensive HiFace mod using a -10db attenuator. There is also information about other HiFace mods.
