I have a Sirius tuner that is fed into my head unit via line level (not fm modulated) and it sounds really good. It is definitely better than FM and close to CD for me. What channels are you listening to? There are one or two that sound pretty bad (Swing Street comes to mind) simply because they are playing really old recordings. But almost all of the other music channels (1-99) sound really good. The higher number channels (100+) are basically talk and don't have the greatest fidelity, but they don't need it. The one exception is Radio Disney (130) that plays a lot of music but doesn't sound that great because it is in the "talk" section since they play commercials.
