Not to correct you, but Sirius actually has 4 birds up. They are called RadioSat-1,RadioSat-2,RadioSat-3, and RadioSat-5. RadioSat-4 was never launched and is sitting on the ground as a spare. RadioSat1-3 are in highly elliptical orbits and their position is constantly changing in the sky. Generally, a maximum of (2) of them are visible at a time over a given point in North America...always at least 1. This is a strange orbit but its advantage is that the satellites are visible at a very high angle from the ground...this is what allows it to clear trees so well. This is really a requirement for mobile work as satellites with a lower angle (inclination) would not be visible from car level when driving in wooded areas. The angle of inclination is often much greater than 45 degrees for these birds (depending on their current position and your location). 60 degrees and up is not uncommon.
RadioSat 5 was just launched 18 months ago or so and is a GeoStationary satellite. It is in a fixed position over the equator and its position in the sky never changes. Because of its position, this satellite has a relatively low angle. In my area the inclination for this bird is around 32 degrees. It is often blocked by trees. This satellite was put up primarily for fixed sirius installations, such as on a house. The benefit is that once you get line of site established to it, you do not have to worry about it dropping out because it doesn't move. The original 3 satellites provided some challenges for these installs as you could have perfect signal when installed, but as the birds move during the day they might pop behind an obstacle/tree etc. The other BIG benefit of RadioSat 5 is that it is using an extremely high gain antenna/reflector and is effectively beaming the signal to the East and West coast...This high signal power is actually much more effective at penetrating leaves and such if they do get in the way. Since the East and West coast have the lowest angles for the elliptical orbit birds (RadioSat1-3), and are therefore most affected by trees, this is a good way to help mitigate that somewhat.
Sirius does an excellent job of getting signal from space and has made some major investments. It is my opinion that this problem they are having HAS to be a ground station issue where the actual stream is generated. I am having problems with skips/audio dropouts with full signal bars and clear LoS to RadioSat5 as well as signal to the other (2) birds likely to be overhead at any given time.
Please note that the above statements are specific to SIRIUS satellites. I know very little about the legacy XM birds.