ipod vs CD vs satellite

I am setting up a system in a boat with high powered amps and high sensitivity compression horn speakers. I want to use the cleanest possible source. The existing head unit has no built in CD player, but it can control a cd changer, Ipod, or Sirius.
Here is the question..........if I download to the ipod from itunes and use the Ipod adapter that avoids the ipod's preamp, am I getting CD quality from the ipod? I don't know squat about ipods or compression. I do know that I hear a big difference in my truck when I switch from Sirius to CD. Thanks!