Huh, well, I guess I’ll have to talk with my McIntosh guy about that. Everything I read on the web suggested or said that center channel audio couldn’t be amplified correctly with a left or right channel from the amp.
I’ve been involved in cinema sound long before home theater existed and my experience is that’s not correct. There’s no such thing as a Left, Center or Surround amplifier. They are all just amps subject to the same ratings and limitations of any other amplifier. The labels of Left and Right or Center are just convenience. You might as well number the amps 1, 2, 3, etc. instead.
In cinemas, before Dolby Digital, we would use 2 stereo amps, one channel each for Left, Center, Right and Surround. Yep, we often used a single amp for the entire surround speakers with judicious parallel/serialization. Back then the Surround channel was 1, not 4 or whatever it is now with Atmos.
So, yes, you may use 1 channel of a stereo amp for your center channel so long as the amp and speaker are otherwise a good match for impedance, sensitivity, power, etc.
It’s sad to waste an amp though so not uncommon to see it bridged if the amp supports it. Note that minimum impedance is usually higher for amps when bridged. Otherwise, if your center supports bi-amping it’s another option for fully using a stereo amp.
Since the invention of HT however the idea of a 3-channel amp to enhance a stereo amp has become popular as well... the idea being that you can power 2 surround channels plus the center. Again, there’s absolutely no difference in the electronics. It’s just a 3 channel amplifier.
Of course, what you should be doing is using 7 monoblocks.... Hahahaha. Just kidding, but it would work too. :)
Having said all of that, the input to whatever amp is connected to your center speaker should come from your HT receiver/processor’s center out. Not any other source. There have been a number of mini appliances in the past to create a center channel from L and R outputs... but I can’t think of a reason to use them in the 21st century unless you are trying to recreate 3 channel stereo from the early recording days.