For bonus points, these iPods have a Wolfson DAC chip, and there is a mod where you could directly connect the output from the DAC to the line outs on the dock connector. The reasoning was that Apple put in a quality DAC, but all the components in the path to the line out were crap! You also had to put capacitors in-line with the signal because apparently there was some voltage coming out of the DAC, and if your headphone amplifier didn't have some way to suppress that voltage, it would fry the amplifier.
There was a audiophile shop called Red Wine Audio that pioneered this mod. They either performed the mod, or you could buy a modded iPod. In addition, they sold the "line-out dock" that had a dock connector with BlackGate capacitors, and copper wiring with an audiophile plug. Back in the day, the mod cost a few hundred, along with the line-out dock, then a headphone amplifier. It could cost over $1,000 depending on the options.
Over on head-fi.org, some experts figured out how to do the mod and posted instructions. It's definitely not for the shaky handed solderers! I took a look at trying to do the mod myself, but soldering those tiny pins on the DAC takes an extra steady hand. They figured out how to mod the iPod 4 and then some subsequent versions. Although I think past the iPod 5, Apple wised up and started using cheaper DACs, as they figured out that putting crappy components downstream of an audiophile component ended up being the weaker links.
The iPod is limited to 16-bit though, so with the modern DAPs moving toward 24/96 DACs, these modded iPods are going the way of ... well ... the iPod! People either go to their phones, or if they want the audiophile experience, they go to the Astell & Kern. I still have my modded iPod tho! Complete with the flash memory mod too. I love it.