If you go from the headphone output of the iPod to an input on your preamp the signal from the iPod has passed through both the digital to analog converter and the amplifer in the iPod. Then the signal is amplified again by your own amplifier and the quality of it won't ever be any better than what the iPod itself is capable of.
If you use one of the cables like Metralla is describing, or one of the standard iPod docks, you are bypassing the internal amp of the iPod but still using its digital to analog converter.
Third option is a device like the Wadia dock which is able to take a digital signal from the iPod. Then the digital signal is converted and amplified outside the iPod.
None of that may matter to you, though, if all you're after is convenience or if the files on the iPod are highly compressed mp3's where the quality of the external conversion and amplification aren't going to significantly change the sound.
