On many occasions, when I had my "Belles" in my large listening room, I noticed this same effect. Many times I would play a mono album, without realizing it was mono and being astonished with the sound quality. Only then to be surprised to see that it was not a stereo album.
Now that I have the "Belles" set up in my study, with a Fisher 500C, I am considering hooking up a center speaker to see what the effect is. I see no pre-senile dementia here, yet!!