Up to press I haven't noticed any major changes in the sound presentation between all the software, which is why I am sticking with BitPerfect. I have just bought my first external DAC, so am just getting used to the differences that this makes.
Part of the reason for not noticing any major difference, I think, is that I do not currently have the best listening environment in which to relax into the music. It is all being listened to through in-ear 'phones (Klipsch X10i.) These sound great, but I do not find sitting up at a desk to be as relaxing as sinking into the sofa and listening through a pair of speakers. (For many reasons there are, currently, no speakers in the house through which I can do this.) So at the moment I am sticking with BitPerfect for the convenience factor. When I finally buy some speakers (which, due to space constraints, will most likely be the Paradigm Soundtrack) I will upgrade my Mac's RAM and trial the software again. I have found Pure Music, though, to be overly complicated.
Still, an independant assessment of the benefits of the various options could prove interesting.
MacBook > Geek Out DAC/Headphone amp > Klipsch X10i/Bang & Olufsen Earset3/Shure E3/B&W C5
The reason for not hearing any differences is because there are none. You will find iTunes to be as good as anything and your headphones would be superior to a great many hifi setups in terms of clarity. More than good enough to determine audible differences if they existed.
That's right, Audirvana will improve the sound somewhat because it does this...
Integer mode: talk directly to lowest level of OS X Audio in the DAC native format.
And this 'memory play' is the best thing about it. (loads, decode and convert tracks completely before playback) Just at least 4GB RAM!
Also puts less strain on your harddiskdrive when playing music for hours.
And if you really don't like iTunes than JRiver might be a good alternative. (less expensive than Puremusic)
[ CambridgeAudio 350A + 651BD + Tannoy RevoSign DC4 + Sharp TV ] [ GDrive 750GB -Firewire800 > Mac Mini -2010, Audirvana+ 2 & iTunes -A/FLAC > USB 16&24bit & iFi Nano idsd DAC. ] [ Dual 505-1 (Ortofon) + Cambridge 551P ]