I use a modified Transit with coax cable attached. It passes everything, including iTunes, bit-perfect .wav files and even MP3. I drive a modified P-3A upsampling DAC with it. The quality of output is as follows:
MP3 - acceptable, but depth and dynamics are compromised
.wav @ 44.1 - as good as the best transport
iTunes - more dynamic than .wav files, but vocals are a bit hollow
.wav @96 - best of all. a bit more dynamic than iTunes and more natural on vocals. Leavs all transports in the dust.