Why does the Sirius XM sound so digitaized/compressed compared to my Nissan NV Van?

[NorCal_MP3]

While CD sound and Bluetooth sound pretty good, the Sirius XM is horrible...I just assumed it was all the Sirius XM signal, but my new 2014 Nissan NV Cargo has only 4 speakers (which actually sound pretty good) and the Sirius XM sounds noticeably better than the Mazda, (this is the previous generation Connect, but about as old as the NB1 units), almost as good as a CD...could it be the DSP? Anyone swapped the headunits with aftermarket and noticed a positive change in XM sound?

 

[MikeM.]

I've never heard Sirius sound good in any vehicle. And I'm not a picky audiophile.