don't discount the source.
Virtually any cable company sends out a great HD signal but how they handle the SD stuff may be another matter entirely.
As much as I hate comcast, I'll give 'em credit here when it comes to signal quality. I can watch anything and still get a clean, crisp picture but, naturally, HD is light years ahead of SD. Now, my buddy in another town with cablevision, well...
then again, I'm using a Toshiba and he's got an Olevia (sp?) but, ultimately, they both are coming out of boxes provided by the service supplier over composite connections.