Well, I'm gunna answer my own d@mn question here :-) I guess XM has a larger number of ground based repeaters which to me would explain why you're able to get that far in to the Lincoln tunnel before dropping ... XM also uses geostationary satellites, two I believe; vs. Sirius 3 sats in a figure 8 shape orbit stretchin' from the southern tip of south america, west bound almost to Hawaii, and as far north as the northern tip of Canada.
I guess based on this I can see why XM could potentially be a bit more reliable than Sirius and maybe stretch a wee bit further in to areas like the one you mentioned ...

Now that I'm looking at XM I should've known .. those birds were built by Boeing ... one of our customers :-) I'm 99% surethe tele units on those things are ours *grin* The sirius birds were built by Loral, we don't touch em with a 10 ...err 20 foot pole LOL! Damnit, I should get an XM discount for the contribution here .. hehehehehehe

 

As an avid music aficionado, my H2's XM1 & XM2 folders are programmed to the following stations;The '60s, The '70s, The '80s, Cinemagic, Watercolors, Audio Visions, Top Tracks, The Groove, Soul Street, and XM165 so that I can listen toBruce Williams (Lifestyle TalkRadio Network)...with the balance of the presets allotted to thoseother stations whichmy wife and son have 'staked their claim' in.

 

20 on 20, Lucy 54, Audio Visons 77, Talk Radio 165, 80's 8 & 90's 9