What do you think of the quality (sound) of XM radio in your car?

I have top of the line Memphis Audio speakers and Top of the line Panasonic radio in my SUV.My local stations sound superb! I just hooked up my XM espress via the FM modulator method.I did all the antenna wiring correct. (used the roof mount antenna and the clip-on to factory antenna method as recommended.The music sounds kind of flat.The highs are all but obsolete.There is no broad rand of frequencies (sound) heard as good as my local stations.Anyone with a very good system ALSO notice this? Is this normal?