Here we are with a new and interesting topic: Sources
According to Head-Fi the source is the first device that sends out an analog signal, such as a CD player, sound card, iPod, DAC or turntable.
But there is an important component in you chain that affects the quality of your music too: the PLAYER.
The Transport is the first device in the chain that holds the data and the source is what sends the analog signal to the next step of the chain, call it an amplifier.
In the case of digital music you need before the source and transport something to translate the stored sound information to them. That's the player, and we'll start from there.
The player is the actual software that will take the music file and send it to the transport.
Many people think that you can use any software to do this, but once you understand that each player has its own implementation there are small variations in which they "translate" the information. This is not a crazy audiophile kinda statement; there are certain softwares that sound worst or better than others. For instance you can detect this quite easy by downloading different players and comparing it with your iTunes or Windows Media Player.
When making A - B comparisons be sure to play with just one variation, so I recommend you to get Audirvana (for OSX) or Foobar2k (fow Win) both are freeware audiophile grade players that can work with almost any kind of music file you throw in it. If you're not familiar with FLAC, ALAC, AIFF files, choose an MP3 song you know very well with a sample rate higher than 192kbps, 320 would be the best choice. After you do this, load it in your preinstalled player (lets call it iTunes) and in Audirvana/Foobar, then hit play in iTunes. Once the song is over, play it in Audirvana/Foobar. I bet you'll notice a change.
Some players sound muffled, dark and diffuse. The way they decode the music is not the best and you can hear it. There are many implementations in players like Audirvana, Fidelia, Amarra, Foobar, Winamp, that let you not only listen to uncompressed music, but also, the non purists, can use plugins, Audio Units, VST, so you can personalize even more your listening experience through equalizers, digital crossfeds, limiters, etc, This is very useful for those who cant but 4 or 5 different headphones, one for each music genere they listen, so you can tweak your gear a bit to get the best out of it.
My personal favorite for OSX is Fidelia and for Windows is Foobar2K, there are many other players in the web you can take a look at this link in Head-Fi for OSX players and this link for Windows players.
Most of the developers lets you download a test version of the software when its a commercial player, please do it, try all of them, compare all the functions and specially the sound in combination with your system.