In my experience its often that the output driver gets saturated. For example if I plug my old iPod into the standard apple earbuds which have a very low imped

In my experience its often that the output driver gets saturated. For example if I plug my old iPod into the standard apple earbuds which have a very low impedance (~25 ohms at 100hz) I can easily push the output stage into clipping. Basically the supply rails can't push enough current and as a result the voltage of the rails sags down low enough to clip the output. On the other hand if I plug in my Sennheiser HD580 headphones with an impedance around 300 ohms I can't push the output into clipping (unless the source was clipped to begin with of course).