I have XM and the cassette adapter sounded like crapola!
I am set up with an FM modulator. The sound quality is 'near' CD quality...the limitations inherent if the FM signal prevents it from being true CD quality.
I don't know if any of the plug and plays offered for Sirius or XM allow for any other connection besides the cassette adapter or the FM modulator. As far as I know, for the best sound quality possible a Sirius or XM ready head unit in the car is required. With these systems the satellite tuner is wired directly to the head unit...although I dont think it is a digital connection like Coax or Fiber Optic, I could be wrong though.
I agree with Todd 100%:thumbsup: