Thank you both for your replies. I love avsforum posters! So helpful and willing to share. And I learn so much every time I ask a question.
So here's what I've found:
1. I set my 5450 to RGB full as Tong Chia recommended. That did make a visible difference. It's still darker than the live, OTA picture, but the contrast is higher. But ...
2. I discovered the blacks and whites test videos in WMC. Guess what? The picture I was complaining about doesn't have crushed blacks or whites, but the one using RGB 0-255 from the video card does. So what does that mean? I prefer my blacks & whites crushed?
3. I found a registry key change that is supposed to do what Foxbat121 suggested (set WMC to full range), however it did not make any difference that I could see. I kept it at 0-255 (value=1), though, just in case.
4. I changed my TV setting to 0-255 from "auto". That was responsible for the crushed blacks & whites! With the TV set to 0-255, and my 5450 set to RGB full (0-255), WMC's test videos still look ok. So, definitely some improvement there.
Even though it is an improvement, it still doesn't look as good as the OTA picture. But I think the difference now is mainly brightness (it's pretty difficult to tell flipping between two live sources like that). I'll probably fool around with the TV brightness on the computer HDMI input to see if I can get it to look more the like OTA picture (which I really like), but without calibration I'll just be guessing. Still, it's already improved from what I had, so that's great. I will probably also play around with the MadVR renderer when I get the time (because it'll be fun
.) If I encounter anything that might be useful to someone else reading this in the future, I'll post an update.
Thanks again, guys!