Ah... the good old days, when you could get up, walk over to the TV, turn the knob and if there was a signal on the channel selected... you would see it.
WMC is nice in many respects, but MS tends to presume to do more thinking for the user than might be needed. I've found that if you tell it you live in a given ZIP code, they give you 'THE' lineup they think is appropriate. As I recall there is some provision for manual addition of stations, but the lineup server wont play nice with the information. I have experimented with 'alternate' ZIP codes and was able to get WMC to display a more complete lineup.
I finally settled on a DIY solution for PVR/DVR functionality:
http://forum.tvfool.com/showthread.php?t=820
Oh, FWIW, signal quality indication in the 50% range is 'on the edge' at best. Usually SQ > 60% is needed for reliable viewing. SQ > 70% is needed for a reliable recording.