Thanks so much to Bob Brennan, htwaits, hernanu, and smarty-pants, all of whom have been kind enough to share useful and relevant info. This is a great forum that I wish I had discovered earlier.
Regrettably, though, all of the tips on how to set the picture format to pass-through, aka Auto-HDMI look great, but I have been unable to execute them.
The reason for my failure is that all of the techniques involve the use of a 'Settings' button that apparently appears commonly on many of the versions of the Comcast Scientific Atlanta 8300 HD remote, but not appear on my remote.
What I have is what the Comcast web page at http://customer.comcast.com/remotes/#download
describes as the "Silver with Red OK-Select Button" and the illustration of my remote, showing the location and functions of all the buttons is here: http://customer.comcast.com/~/media/support_comcast_com/Remotes/Manuals/Silver_with_Red_OK_Button.ashx
As you can see, there is no Setup button.
I can get into a status screen using my remote, by first pressing 'Cable' and then holding down 'Guide' and 'Info' simultaneously, but all that brings up is a status screen reporting the state of of things like memory, parameters, clocks etc. By using the arrow keys, I can page through 27 pages of such summaries - none of which have anything to do with picture format or how I can reset it to pass-through or auto-HDMI.
Make no mistake, I truly appreciate all of the good advice from everyone. I just appear to have a different type of Xfinity remote. I'm thinking that I may have to resort to (shudder . . . ominous organ music) calling Comcast. The horror. The horror.