Steve, thanks for the reply. It is now working.
I continued diagnosis. Before I posted, I hadn't tried a different speaker on the center channel terminal yet, so I did that and it worked. So I put the CC speaker back on that circuit and its been working ever since. I didn't see any obvious wire break, strands or reversed polarity, so it seemed very strange that it worked every time on another channel. Its now plugged in and has been working great for a few hours now. However, the wires are quite old so maybe its time to replace them (any excuse to upgrade...).
I'm now happy. I've run through auto MCACC again, and the CC is much brighter in this setup v. my old AVR, but at least I know how to play with manual MCACC due to this troubleshooting. At least I'm not forcibly becoming a member of the 'no center channel club'.
Thanks again for the help.