I've been using my Direct TV receiver in my main system as the Music Choice they used to offer had quite good sound quality. Similar to CD, in fact. I was really bummed when they switched to carrying the music channels from XM. The sound quality is dreadful by comparison, but not worse than typical FM through a Mid Fi system, I suppose.
As it happens, I have Sirius on my secondary (office) system through a Kenwood DT7000 receiver. The music quality is definitely worse than CD most of the time. They do, however, claim to use some sort of dynamic compression so that the sound quality may improve based on how many football games they are carrying etc. Also, depending on the channel in use, the sound approaches acceptable, but never what I'd call HI-FI. I find that Sirius is quite acceptable in the car with a Sirius One receiver which just uses an RF modulator to get the signal to the stereo. It works fine though in that environment. Bottom line for me is that Music Choice on Direct TVwas a significant loss in terms of music quality. I would not consider either Sirius or XM a real high end source, but of course, I'm not using it that way. I'm a Stern fan, btw, so the Sirius will stay for content, but not for its ability to provide high-grade sound.