In case this may help someone else:
I tried connecting direct from sources to the tv. HTPC, no go. Cable box, yes. Blu-ray, in and out.
Found the problem though...
I had the HTPC screen res. set to 3840x2160 (recommended by Windows). Why it said that? I think it saw, through HDMI, that the new 4k tv was hooked up (through my Denon AVR).
I switched screen res to 1920x1080. Boom, fixed! HTPC output looks great AND blu-ray output is now rock solid. Cable signal still rock solid, looks great.
The last issue was to get the TV smart hub apps (Netflix, youtube, etc.) able to send the audio BACK to the Denon AVR via the HDMI ARC connection, othewise I would not have sound using those apps (tv speakers are OFF).
There is a menu setting for audio output on the tv setup that must be set to 'Receiver', but that choice was disabled. Found out I had to set 'HDMI Control' to 'ON'. It was off.
Now, everything is good to go.