[William Sommerwerck]

You should hear what happens to mono recordings when a bit of
stereo ambience is added to the room. The improvement is drastic.

I think you forgot the parentheses around improvement. I've yet to
hear any decent mono recording improved by adding stereo
ambience -- and that includes pro attempts.

Because -- as I repeatedly stated -- you didn't hear it correctly done. You
don't add the ambience to the recording, but through additional speakers.
The results are much different.

I will add one qualification... The recording has to be reasonably good to
begin with. Really old mono recordings sound rather odd with stereo
ambience -- though the oddness is more aesthetic than acoustic.