Wish I'd seen this earlier. I have 2 Sangean HD radios, though not the type you have. I have an HDR-16 and an HDR-18, both of which I purchased within about the past year. The 16 is a portable, and while running on batteries it'll pick up the stations fmfool lists down to about -55dBm. Some down to -65dBm come in, at least while I'm upstairs (2-story house). There are a few stations fmfool lists within that range that I cannot detect there's any transmission there at all, while others in that range come in with only a small to moderate amount of static. This is with the telescoping antenna, of course. Unfortunately, none of the stations in my area with that weak of signal strength at my location broadcast an HD Radio signal.
I lived in Norman for a short while, years ago, so I could have told you you've quite a job to do if you want to pick up Tulsa from OKC clearly. I can't imagine HD signals would make it. Tucson is a similar distance from me as Tulsa is from OKC (maybe a bit further than that), and Tucson FM stations are essentially not there.
I _am_ curious, though, to hear more of what you think of that receiver as you gain more experience with it. I don't have a home stereo system any more (to my disappointment), but a receiver like that HDT-20 might still be useful to me at home. Or I could just go on using my HDR-16 but get some speakers for it.
I'd also be curious to hear what you think of the AM side of the tuner, if you ever use it. I don't know (or don't recall?) whether the HDT-20 has a built-in ferrite antenna. I'm quite happy with the HDR-16's AM tuner + built-in antenna, and I'm _very_ happy with the HDR-18's antenna + AM tuner.