When I installed Sirius in my car, I was using FM modulation. When I switched to line-input, it sounded much better--better high frequency response was the most noticeable improvement. The FM modulator I was using is the Kenwood KCA-R70FM to which is connected the KTC-SR901 Sirius tuner.
Neither of those units has line-out so I had to add it by soldering RCA plugs to the appropriate pins on the interface cable connector (on the FM modulator).