Clarification please...
What are you expecting to pick up with the tuner in the S-101?
What type programming do the "3 good coaxial cable sources" deliver? the fact that you state that three TV's work well with them causes me to think they are TV sources.
I think the S-101 tuner is designed to pick up FM, not TV, so if you are connecting TV cables to the Denon, there might be your problem.
You might try an FM antenna and then get back to us.