You sit close, you have good eyes, and you've done good tests; sounds like an equipment problem. If I were you, I'd get a second opinion ("hey friend_x, does this channel look like HD when compared to this channel?" ), and, if they think as you do, call your cable guy out. You shouldn't be expecting HD to change your life, but you should see a sizeable difference. Or at least SOME difference. The biggest thing to me is that you're saying you see no difference at all. If you just didn't think it was a big difference, then maybe I could argue that your expectations were out of whack. As it really does look the same, though. . .I bet you've got a problem.

Good luck.