It actually comes down to the SiriusXM receiver that is being used - I've witnessed the built-in sirius/xm on on the latest GM platform (a $100,000+ Cadillac) sounding like AM radio to immediately sitting in a better-than-apple-streaming quality rendition of the exact same channel on an older lexus a few minutes apart...

The mobile xm receivers (like ipods) that they used to sell also had very good quality and I never noticed any quality shortcomings even with good headphones.

I think the "high" quality stream is 256kbps/16k which is fairly high compared to most streaming services that come in around 128/160.