It is widely known that the optimum signal strength is achieved when the antenna is facing a southerly direction, which I assume you also know since you are asking about a signal being able to penetrate double pane windows from the North.
I find that when the unit is hooked up to the indoor antenna, generally the signal is very good, even when facing a direction other than the South, so long as it is not facing anything but a window. Blinds may, but shouldn't affect it much, unless they're made of some really dense material, which I'm thinking yours are since they were a mention....
XM reps are right in saying "go head, try it", becuase they (XM) already have their asses covered by stating that the antenna should be facing in a southerly direction, and that anything else is Caveat Emptor" But you know, you can always do a month long trial, which within that time you can return the unit, and only have subscribed for $12. No big loss there.
Personally speaking though, if it were not for a certain talk show on XM, I'd not subscribe for the music. The audio quality is absolutely atrocious when heard through a decent home system. Hell, even my pair of cheap Optimus speakers through my old Onkyo TX SV515 Pro exploits the dreadful results which are brought on by low bitrate compression. Even their "premium" (see: 48 kbps) channels still sound like crap, no matter what kind of magical synthetic compression schemes they try and toss at it... Highs are pretty much non existent, and when heard, are sibilant enough to derail any mellow mood. Mids are flat and pretty much the same as the highs, not there. Bass is disgustingly muddy, and lack any depth or punch.
I'd not recommend their service if you really enjoy "listening" to music. At the same token, it's a great medium for finding new music, and getting local news/sports. If you don't care too much about audio quality (what'cha doin here then ?!) and want it more for the curiosity etc, then it's probably just fine.
And no, Sirius is no better. Worse, if anything, or at least they used to be. Seems that XM has really put its priorities in the crapper. More channels, in order to out do Sirius, has chomped away at their bandwidth so much, that what used to be almost acceptable (perhaps used to sound more like 96 kbps), has turned into "how the hell are they getting away with this?"
Of course, you may already have the sub, and just haven't hooked it up yet. I couldn't tell from your post if you owned a unit yet or not... My sub runs out in a few days, and I'm not sure that I'll be renewing it since I'm actually able to download the talk shows I listen to . It's just nice being able to listen to them whenever I want on the fly... Have to get MY priorities in order now I guess.
Good luck bro...