most aduio devices only run to x volume, till the start to distort.. the internal amp in the mp3 player is only meant to be pusihg the small earphones which only require nominal power.

so when you turn the volume up on the mp3 player, it will distort, because youre trying to get more out of it than it can produce safley. and since its prducing a cleaner signal to teh tape deck, the tape deck, which will be louder than the mp3 player, wont distort easier. But if you crank the tape deck it will.. lemme know if this makes sense