There is a less known feature of the Mac that may potentially degrade sound quality. The Mac has an internal sample rate converter. I don't know if it is implemented in hardware or software but believe me, you don't want to use it in either case. The problem is that it is automatically engaged if the sample rate of the source material is different from the setting of the audio output device. For example, if you are playing a 16/44.1k track from iTunes and the Mac's audio output device is set to 24/96k, it will up-sample your track and output 24/96k signal to your external DAC.
The only way to disengage it is to make sure the sample rate of the source material is the same as the sample rate setting of the audio output device. To check that, you need to use the Audio MIDI Setup program which is usually located in the Application/Utilities folder.
Once you open the Audio MIDI Setup, click the Audio Devices tag at the top and look into the "Properties For:" drop down box. As soon as your USB to SPDIF converter is connected, it should show up as one of the selection. Now select that and look over to the lower right corner and you should see some controls show up in the Audio Output section. Now you can change the sample rate to match.
