Regarding XM/Sirius quality, here's some data from '08 I found:
Satellite radio bit rate channel by channel - Hydrogenaudio Forums
For reference, 64kbps is roughly considered FM quality.
So, if current XM/Sirius bitrates are roughly the same as these '08 values, then I wouldn't be surprised that FM sounds better than most of the XM/Sirius channels.
This should be independent of volume level.