"…the iPhone 5 is a wonderful high-fidelity audio source. While publications funded mostly by advertisements from makers of expensive cables, power conditioners and outboard DACs don't want you to know this, the iPhone 5 is a better audio source than most DACs will be when connected to a computer or CD transport. The only difference is that the iPhone has a level 6 dB lower than a proper CD player, but the iPhone still has more output at 1 V full-scale than some outboard audiophile DACs!"
"There isn't much difference between different iPods, iPhones and iPads; all pretty much have ultraflat response, near-zero output impedances and deliver 1 V RMS full-scale. People who hear huge differences in bass or treble aren't comparing them competently, or are comparing the internal speakers or something else."
Source: http://www.kenrockwell.com/apple/iphone-5/audio-quality.htm