I use Decibel. I don't have experience with PM or Amarra, but I use Decibel for two reasons:
1) It's very inexpensive. Yes, the interface is basic, but it does everything I need it to do and it sounds great.
2) Decibel can be set to play tracks from memory. Data for the track is buffered into RAM and played back from there. This is important to me as I use it to stream music over wi-fi from an NAS in another room to an iMac connected to the stereo. Since the track data is buffered, I get gap-free, glitch-free playback over wi-fi.