Jumping into an old thread for anyone who still cares. My info comes from postings over on the XM fan board, where they track such things. As always, I'm no expert, could be wrong. etc.
- Everyone who says Sirius and XM are bandwidth starved, highly, highly compressed is in my opinion...correct. That said, Sirius and XM, despite being the same company for the last half dozen years, use different audio compression schemes. As noted by another poster, XM uses AAC+, (I believe it's also called HE-AAC) while Sirius uses a different codec, whose name escapes me at the moment. Among XM fans at least, there's a consensus that AAC+ sounds better.
- Probably more important is the amount of processing the signal undergoes after compression. From what I've read - and my own listening bears this out - Sirius pushes processing hard; it's like FM loudness on steroids. Everything, from the quietest to the loudest passage, comes out the same. It will drive you insane, and not slowly.
The XM fan board reports that as of mid-year last year, it appeared the XM side dropped most/all post-processing, resulting in markedly better sound. I have Sirius in my car, built in, but am encouraged enough by this news that I've acquired an Onyx Plus XM receiver and plan to install it to see if I can hear a difference.
Finally, in my little town Sirius now has significant dropouts. I believe this to be related to the decommissioning of a couple of satellites, but don't know for sure. An early test with the Onyx, which would rely on the XM satellites as I understand it, was not encouraging.
s.