The frequency response sounds a little flat in all Satellite radio systems I have heard. Audi, BMW, Nissan. It's so bad in some cases, it almost sounds like a mono signal with lame speakers.
The Audi boards are littered with folks unhappy about the sound quality of XM and Sirius. I don't think it's specific to Sirius, or Logic 7. I think it's just the inherent quality of the stream. It shouldn't have to be that way, since it is all digital and a relatively high bandwidth capability, but for some reason it is.