The quality has degraded slightly since the merger...here's why.
Before when they were seperate, they weren't cramming as many channels in to the same bandwidth and everyone had a little more breathing room..but you're still talkin 64kbps AAC-SBR at best. (SBR, in case you don't know, is Spectral Band Recreation...only a "fingerprint" of the high-frequencies is sent...the actual audio spectral bandwidth is much less)
Since the merger, every one's been squeezed even tighter. The quality of Sirius/XM is fine for your car....however, I will NOT listen to the actual service on any kind of hi-fi equipment. The only way I'll listen to sirius/XM on a good hi-fi is through the rebroadcast on DirecTV/Dish which, while the same audio, comes from a much higher bitrate mpeg stream than what they're pumping to the dedicated units.
Ok, as far as antenna aiming...I do not have Sirius..but let me explain XM.
They both work in the L band....this band CAN penetrate thin layers of plywood and roofing materials. Clyde is half right....however I will tell you my XM radio has worked...indoors...with the antenna in the middle of the room not even close to any windows. It will penetrate better than you think, but not as well as say FM reception. Windows are good...attic is good...but sometimes, you can get a good signal if you "aim" it south. This does not include terresterial repeaters you may/may not live near. I happen to live a few miles from a MASSIVE repeater used to cover a wooded area (last spec I read on the thing said 50k watts)...so as long as i'm facing the general direction of south, i get reception in my bathroom! XM sats are in geo-sync orbit.
Sirius...I don't now much about...does it have a repeater network? can't tell ya...what i do know is thier three satellites are in a LEO (low earth orbit), so they're not stationary...they move...and for the most part unlike the geosyncs that are typically around 40 degrees above the horizon...these suckers pass right over top of your head.
there is no radio on the market that uses both systems at once..i believe they have some planned..but it'll be probably a major software/hardware overhaul. they might have decided not to...they might have decided to do it. I don't know...but when they get those units out...reception will be much more reliable for a crappy audio stream to begin with.
It's a shame we don't have DAB over here...DAB actually uses I believe either layer 2 or layer 3, but, still, they too are slightly overcompressed.