One point, I seem to remember reading somewhere that the quality of sound from the raspberry pi's line-out isn't great so an external sound-card is recommended. You might want to check that out.
Depending on your amp inputs and what you're playing music on then a chromecast could be a nice easy solution. Similarly, a bluetooth adapter (particularly if your phone has Apt-X).
PiMusicBox is meant to be good http://www.woutervanwijk.nl/pimusicbox/ I've not used it yet but it's on my list of stuff to play with.
One point, I seem to remember reading somewhere that the quality of sound from the raspberry pi's line-out isn't great so an external sound-card is recommended. You might want to check that out.
Depending on your amp inputs and what you're playing music on then a chromecast could be a nice easy solution. Similarly, a bluetooth adapter (particularly if your phone has Apt-X).