I have a 2010 SRT with the option group II. When Sirius is on the music sounds tinny!!.. I don't know how to describe it. I f I play my iPod it sounds ok.
Something else to keep in mind, XM/Sirius allots different bit-rates to different channels (it's obvious to a trained ear - I have no factual evidence to back this up except years of pro-audio experience & 20hz to 20khz hearing). If your favorite station falls under one of the lower bit rate channels, they often sound worse than AM.