I bought into Audirvana a few years back and agree that it is indeed a noticeable improvement on iTunes native playback ... which is a bit pants really...
As I understand it, AV over-rides the Mac's inbuilt audio gubbins and uses its own audio codec which accounts for the improvement but to be fair to iTunes it lacks all the additional bells and whistles such as CD ripping, file conversion options and data tagging. Having experienced the total wipeout of some of the other 'free' media utilities that made the MacOS such a really useful platform (such as iPhoto, iMovie and iDVD) I am grateful that iTunes has survived as a relatively useful programme.
What surprise me is that Apple haven't bought AV out yet and incorporated it - they are usually quick off the mark to swallow up anything that swims in their waters ....