I've had my Polk xm for about two weeks now and the antenna is behind my right speaker not near the window at all and I'm getting full signal strength. Your geographic location may get you different results.
Polk Audio XM Radio Reciever A Winner
I just received the new Polk XM radio receiver. I planned a series of a/b listening tests with my old XM radio. It only took about 10 seconds of listening to realize how much the sound quality improved. Highly recommended. It is not quite cd quality but far better than the standard fare. FYI my system is Rotel preamp, Adcom mono blocks and Magnepan 1.6s.