I was not happy with the sound of my iPod running through the y-cable into the factory iPod integration for the stereo in my MCS. It was significantly distorted (you could hear the music but it had background distortion that was clearly audible), despite playing with various input levels, controls, eq, etc.

I noticed that the sound quality was better when I just plugged in the iPod via the headphone jack, so I tried plugging in the usb side of the y-cable with a aux cable from the headphone jack. I was very surprised, the sound quality was much much better, and it allows me to get the charging and control function through the radio controls (um, limited tho it is... lol)

I (obviously) suspect that the y-cable is a problem and will replace it, but am wondering, is there a difference in line level output between the iPod 30pin socket and the headphone jack? It finally sounds like I expected it would, qualitywise.

 

There is a difference in the Line Out and Headphone jack.

The line out is a non adjustable output and is set to an industry standard level.

Sound to the headphone jack goes thru another amplifier that has adjustable gain and thus allows volume control to the earphones that are normally used there.

Your experience is quite odd as most people will tel you that the line out is the one that should be used for the better sound quality. But if you "Y-Cable" is bad then it is always possible that what you are hearing is true.