Sure! When you launch Audirvana Plus it grabs hold (depending on your settings) of Audio MIDI Setup, the volume controls, and the iTunes interface. The first is so that bypasses Core Audio and will always play files at the native resolution, even if you change it on the fly (one used to have to quit iTunes, change it in Audio MIDI setup, and then relaunch iTunes). The second is so that it controls the volume to avoid bit loss and breakup from digital attenuation -- you also have to option of bypassing attenuation on the computer side and doing it using the DAC. And the third is so that you can still use the iTunes library to control your music with all your playlists and ratings intact. There's a noticeable improvement in sound quality; am not qualified to compare it objectively to Amarra and Pure Music, but I find it works for me, and I like the fact that it's unobtrusive and gets outs of your way. So when I want to play music now I launch Audirvana rather than iTunes, although it's the iTunes window which comes up. Tried going back to direct iTunes playback -- like many things, you notice it less on the upgrade than when going back; everything feels dynamically limited.
Hope this helps!