I think the distortion is because of the low quality amp built in the ipods. I plugged the same earphone into my mac (I added ultimate ear's airplane attenuator, so there is no hissing noise) and played the same track on iTunes with Bass Booster, and Acoustic settings, and there were noticeably less distortion. I wonder if headphone amps can eliminate the distortion.
I also found a song, "Infinite White" from the "Transformers: Revenge of the Fallen [Original Score]" (Yeah, I'm a Transformers fan) it has a lot of bass, and also a female vocal. So what I found out is that the female vocal is distorted around 0:48 (even without EQ).