I just downloaded the trial version of Audirvana Plus (the newest version 1.5.12). In the past I've tried Pure Music (a couple of years ago--didn't like the interface, and it was just awkward to use) and more recently I've been using BitPerfect. I use iTunes, and I have tracks with various sample rates (mostly ripped CDs, but some HD tracks 24/96 and 24/192, etc.). I liked BitPerfect just because it would switch sample rates on the fly. I didn't notice much of an audible difference with either it or Pure Music--I just wanted the memory play, hog mode, and auto switching of sample rates that iTunes alone couldn't handle.
It seems that with every OS X update, though, and/or with iTunes updates, BitPerfect would sometimes stutter and crackle when switching tracks. I tried all of the suggested preference changes and fixes, and although it would work fine for awhile, it was never completely glitch free when starting new tracks.
With Audirvana, I'm getting all of the above advantages; I get to use iTunes like I always have, I can use my iPhone or iPad as a remote, and there are absolutely no glitches or stutters when switching tracks. Only silence. If I'm playing a 24/96 track, I can double click on a 16/44.1 track on the fly, and after a second or two of silence, the new track will begin without issue. (Not that I hop around that way when I'm listening, but if the software can do that, I'm happy).
And, in addition, I do hear, for the first time, an audible difference with Audirvana. Everything is just a bit more spacious and open. It's a slight difference with some recordings, and quite dramatic on others. I'm going to spend some time with the trial version before I buy the license code, but so far it's looking good.
Also, some info that might be helpful to others: I've just installed the newest Mavericks (10.9.2), along with the newest iTunes (11.1.5) and Audirvana + (1.5.12), and everything seems to work together very well so far, after a couple of hours of listening and playing with the settings.