different Panasonic Plasma's

Hi Bob . . . I just use the digital cable box, or the satellite receiver for DirecTV/dish. The latter usually has a terrestrial ATSC tuner in it as well, for an off-air backup during heavy rain.

If you really need just an off-air ATSC tuner, they're pretty cheap. Having a separate monitor and tuner is still nice for i.e. a typical setup with a high-end surround receiver - the receiver generally has some HDMI routing and transcoding for standard-def signals, so just one cable can go to the monitor. Then all of the input selection for audio and video together can be done right on the receiver.

Yes, a digital cable box functions in effect just like a stand-alone component tuner . . . conceptually very similar to an audio tuner. It provides all the on-screen program guides and channel selection.

As far as the audio goes . . . all of the industrial monitors have basic audio source-switching and preamp functions, as well as a little (i.e. 7 watt) stereo power amp to run some optional accessory speakers that can bolt on the side. But for a system where you want high quality audio . . . the audio sections of ANY mass-market television or monitor should be avoided, as they're typically very noisy. So usually, some sort of receiver or processor (the audio/video correlary to an audiophile line preamp) is used for audio decoding and preamp functions, and for video source switching . . . the latter of which is passed to the monitor for viewing.