thanks for the answers. The thing is, that the volume in the large headphones is just fine (because impedance is 32 ohms, which is the same as the small white buds), so in this sense I don't need an amp to make it louder. What I am concerned about is will it have some bad impact on the internal components.
I think the headphones you need an amp for have higher impendance, thus your ipod cannot make them sound loud enough.
I am just thinking here:
If you have an amp for say 400W and you connect a 600W speaker to it, this has no bad effect on the amp itself: it just cannot reach the 600W. This should be true for the iPod as well. The headphones would just never reach their max volume (which is fine, because even at 60% of volume, they are already extremely loud).