I load up my iTunes library on my iPhone from a Terabyte of Apple lossless files on my MacBook Air home computer's external hard drive. I usually carry about
10-15 GB of music, then I use a iPod Line out dock (which extracts the line out signal after going through the onboard DAC, by bypassing the crappy internal
amp on the iPhone. The plug has a standard iPhone/iPod/ipad/itouch 15 pin jack which plugs into the bottom of the phone. The other side of the LOD is a "mini"
or 1/8" plug that will fit into any headphone amp line in. Headphones are driven by the external amp.
This enables you to use the DAC conversion on the phone and bypass the "average" built in Apple amp, and use a superior amp with a superior pair of headphones.
As long as you use 320kbps or even better yet Lossless ALAC (apple lossless) music files, the music will sound vastly superior to just using the jack on the iPhone.
If anyone wants some pictures of some cool setups, ask me and I will host them. I have headphone amps that are smaller than the iPhone's L X W, that will band to the back of the phone and make the difference between background music, and serious music listening on planes, airports,doctors waiting rooms…anywhere you are really going to spend some quality time with your music. Not made for the gym. Plugging right into the jack is fine for the gym or on Public Transport.