@BaaM's claims (in fact, Damien Plisson's own claim) about Audirvana sweetening the sound is absolutely accurate. It's not that Plisson cannot provide bitperfect output – he can. Audirvana has been on the market for years. To be able to differentiate from other players, Audirvana has to either add features which others don't have or Audirvana has to be cheaper or Audirvana has to offer "better" sound. Damien has chosen to offer "better" sound. In my opinion, he's been quite successful.
Swinsian, Calibri, Decibel, Fidelia all sound identical on music which they can play back at native resolution (Decibel doesn't handle all bitrates of DSD). I haven't tested Bitperfect only because I never use iTunes but from repots it falls into the same group. They are all bitperfect in standard configuration and hence identical. Audirvana sounds different, arguably "better".
What's special about the Audirvana sound is slightly greater stereo separation, creating a "wider" soundstage and more air/atmosphere. There's also a tiny boost in a key treble range (my guess is around 8kHz or 10kHz), giving "more detail" or resolution to the Audirvana sound. But whatever the Audirvana is (and
@amirm likes the special sauce in this case, albeit he usually wants bit perfect), it's not bit perfect.
BTW, I'm not an Audirvana owner or a fan, just own most of the competitors and have trialed intensively several versions of Audirvana. Personally, I decided that I didn't like the price and the constant paid updates (competitors do not do this; Swinsian was a single €21 purchase, Calibra a single €8 purchase, Decibel a single $33 purchase, Fidelia a single $39 purchase. Audirvana is €98 now with €68 for a .5 version update! Audirvana is on v3.5 which makes about €310 laid out for buyers who started with v1. Audirvana is also only two activations (hate this, I have three Macs I use often and another two I use occasionally).
Outside of cost, I didn't like the playlist functionality (auto-updates to my folders didn't work, filtering a library is an way to interact with music). In contrast, Swinsian offers splendid, Mac-native playlist functionality. Create as many playlists as you like. Playlist interfaces are lightning fast, technically detailed as you'd like with a native Mac look. It's easy to view and easy to update files whether you add new metadata or artwork externally or internally. Swinsian will semi-automatically fetch you album art itself from Last.fm, MusicBrainz, coverartarchive.org and a couple of other servers automatically.
On the plus side for Audirvana, the DAC support and exclusive stream is rock solid and provides the most detailed information about the connection of any of the OS X players.