the distortion is most probably due to the fact that you are amplifying the already amplified signal .I pod does not have a dedicated line out 3.5mm stereo jack . if you use an amplifier by connecting it to the 3.5mm jack of ipod, the amplification is done on the already amplified signal ( done by the ipod internal ampliier ).
you will have to get a lineout dock cable (commonly known as "lod" ) which takes out the unamplified signal.