With mine I have nicked the audio lead supplied with my brother's Tom Tom 520 to plug in my iAudio MP3 player and was surprised by the hiss not only when it was plugged in but when the MP3 player wasn't plugged in, there was a lot of buzzing noise - is this normal or is it probably the lead?
I eventually managed to get a reasonable sound by dropping the AUX volume to -2, with flat EQ and full volume on the player, then on the amp I use Bass +1, treble -1, Mid 0, at about volume 16 on the stereo.
Have I got a duff lead if it is buzzing with nothing plugged into it, as I was surprised that I needed to reduce the Aux volume to negatives when the posts above are saying they use either +5 or the default zero? Even at zero, the hiss was absolutely unacceptable, though it could be largely eliminated with road noise and the player being cranked up to full volume (which must sap the battery power and not really be good for it, though I was impressed that it didn't seem to clip at all - excellent quality machine I must say).
Cheers
Rob