Squirrelly, the reason players like Amarra make your music sound better than iTunes is they are typically purpose built to do one thing and that's to play great sounding music. The core audio function on the Mac, which is what iTunes uses for playback, is also busy dealing with every other alert and interrupt that may occur. Amarra bypasses core audio and our Sonic Studio Engine plays back your music, it does one thing and does it right.
Darcilicious provided a laundry list of features that the majority of are common to all players. Each Player has unique features of their own such as Amarra's ability to play FLAC files natively, no conversion necessary. Most of the features don't have alot to do with sound quality directly, they are just nice to have features. For instance Amarra can play back 384k files, but unless you have 384k content and a 384k DAC, the feature has no value.
The bottom line is how efficiently can your music be moved from it's home on a disk somewhere to your ears. I know that Amarra does that more efficiently and effectively than iTunes can and the result is better sound across the entire music spectrum.
Give the trials a shot, they're free so you have nothing to lose.