WOW! Hope I can answer correctly... will do my best:
Main TV - YES we get all our locals both in standard and HD broadcasts, but only the HD on the HDTV of course.
Yes, it has a ATSC Tuner, what I call a "True HD TV" meaning not "HD-Ready", etc.
Yes, My HD-Ready Sony (and all the other analog TV's) operate as you stated, you are GOOD! Want to come over and connect me up?

Just teasing of course.... I am a Grandma 4 X over.
QUESTION: Is there a HI-DEF modulator available that will be equal to and/or replace the one I have? AND would I really need one?? Cost???
QUESTION: In order for my Sony HD-Ready TV to gain a ATSC tuner I would have to directly connect (via GRB cable) my HD20 receiver to the Sony, correct? I asked the DTV install guys about that, but the satellite cables are too far away or so they said. Unless there is another solution I don't know about. Please enlighten me.
I think I have covered all your questions and added a few more of my own.
Let me know if you have anymore questions.
TexasMama
AKA TexasGrandma!
PS: Not sure how the antenna is connected to the modulator; I mean I am not sure if it is a separate cable directly FROM antenna down to modulator OR some other "fancy cable/connections footwork" going on up in the attic I am not aware of. Does it make a difference? Hope all this makes sense.
TM