I've just read the review of the Pure Music software for MAC OS X in HiFi Choice and an currently testing it. So far I am bowled over by the improvement.
I am comparing the same track from my Marantz CD6003 via Pure Silver Audio cables Vs. Mac Book Pro, Apogee Duet, Apple Lossless via Cheap Copper audio cable.
The sounds is notable better via the Mac Book, which is a shock to be honest. The sound is more controlled and over all it sounds of much higher quality. It is upsampling to 96Khz which is a limitation of the Apogee Duet. I wonder how better it would be is silver audio cables and a 192Khz DAC.
Seriously worth a demo!
The only downside so far is the naff interface, really looks it came from Windows 3.11. lol