To explain, you already have a headphone amp in the iPod technically. Most acoustic transducers (speakers, headphones) require an amplifier to produce sound because the audio signal delivered from the source device has much lower power. You need an amplifier to increase the power of the signal so that when its sent to the speaker, there will be sufficient power to make the speaker play at a reasonable volume. Even your iPod default iPod earbuds need an amplifier thus there is already one built inside the iPod.
When people say you need a dedicated headphone amp, it's not to say you don't have one technically capable already, it's to say you don't have one made to power larger headphones that usually need more power. It's also to say you don't have as well designed of one since the one in the iPod needs to fit into a small space, there are compromises in it's design.
Optimally, if you are attaching an external headphone amplifier, you should bypass the built in amplifier in the iPod which will be wired directly to the iPod headphone jack. Basically you should not plug your external amp into the iPod headphone jack. This is double amping. You will have 2 volume controls working in the chain, the iPod and the headphone amp's. For many reasons, this is bad. To bypass the build in iPod amp, you need to get a line level (line out) signal from the iPod. Usually this can be gotten from the dock connector since various iPod accessories like iPod compatible speakers and cars use the line level output as well. If you get a line level output and pass it to the external amp, you will notice that the iPod volume control no longer works and only the amp's volume control works. That's what you want.
