My CD is clearly the best sound, closely followed by iPhone with Aux cable, and a distant third is using Bluetooth with the iPhone. I find a very noticeable loss of definition of the low frequencies with Bluetooth. Satellite and FM are surprisingly poor. AM is a little worse than FM/Sat. I found this to be true with both the original basic system, and with the new bi-amped system with sub and new component speakers in the front.
It is rare that I purchase something that surpasses my expectations, as this car has. I like it.