I have tried the trial versions of Amarra and Amarra HiFi, Decibel and Audirvana Plus, and ended up purchasing Audirvana Plus.
Before using those players I used Songbird for about a month, and although it offers the same amazing song management as iTunes, it doesn't offer its controls nor could I find the amazing bump in sound quality I was hoping for.
Decibel was OK, but starting-up and quitting took a long, long time, as did loading new .flac files. The sound-quality was a major step-up from iTunes though, and the app is generally very stable, apart from sometimes - for unknown reasons - pruning my entire playlist. The interface is very clean and intuitive, but the lack of music management like in iTunes was a bummer, which is a problem that could have been fixed by offering an iTunes integrated mode, which unfortunately isn't present.
Amarra and Amarra HiFi were nice players, but as a lot of you have already mentioned: their interface looks very outdated, and it just couldn't get used to it. I also have a distinct feeling I'm using a toned-down, not-all-you-can-get-version when using HiFi. But their sound quality was very nice, and although I am definitely not audiophile enough to hear major differences between Decibel and Amarra I could tell it was slightly more to my liking.
Apart from the crappy interface, the app did start quickly and closed down quickly, and runs smoothly.
I then auditioned
Audirvana Plus, and for some reason I really like the user interface, which really gives me the feeling of dedicating myself to listening to music, rather than just using it - like iTunes - to play in the background while I'm doing other things on the computer. The iTunes integrated mode takes a few seconds to start-up, but once it is playing it is very stable and runs smoothly. Importing new .flac files is very fast, but unfortunately it only keeps alphabetical order per import, rather than for the whole playlist, so I now have to scroll down to the end for the other Coldplay-songs I've imported. This is really the only thing that bugs me, but to overcome the problem I prune the playlist and re-import all .flac files on my computer again.
Between Amarra and Audirvana I couldn't hear a discernable difference in sound quality, and just like Amarra, I really like how Audirvana presents some of my favourite songs to me.
Furthermore the app seems to be very stable, and starts-up and closes down very quickly.
After about a week, I purchased a license for Audirvana Plus, and I have not regretted it since.
For $49 it is a stable, great-looking player, and you never get the feeling there are any compromises in features or sound quality. Furthermore I feel the iTunes integrated mode is just a tad easier to get used to than that of Amarra.