Thanks for the reply.
Weirdly enough, the issue I described stopped on its own -- so I can see the output screen just fine again. However, I know this won't last, so it must be an intermittent problem that's leading down the avenue of a burned out HDMI board.
We have been running an Onkyo TX-SR605 since it was released (so circa 2007/2008 or so) and it has powered two different systems, first in an apartment we had and then in a house we moved into. It actually does everything we need, save for the height formats of course (Dolby Atmos/DTS:X), as it...