I've tried an unofficial Amarra Demo on ML and it was a bit buggy. Fidelia is a bit buggy too, but only if you try to create a library from files over the WLAN. Once set up plays well. Audirvana and Decibel behave better.
Regarding sound, once everything is set up for "best" results, all they're quite similar listening with headphones (007, 009 and HD800) using the DA-11 on USB. Maybe on long term use I would be able to tell what I like better, but I'm not sure I really want/need to keep comparing them. Same could be said if I used speakers on that system, but it's too darn hot for tube amplifiers here.
I think I'll end up getting Audirvana+ (I like the interface a bit better than Decibel's, but this is also good an inexpensive), but I wouldn't mind joining a group buy if Amarra can be purchased at a sensible price.