[Franco]

P.S.:

"First of all, stereo absolutely has a center (or "middle") channel*.."

Uh, no. Stereo has a "central image", which is totally dependent on the only two channels in a stereo system, Left and Right. Shifting either speaker will change the central image and it will disappear with the absence of either channel. In case this isn't clear, you can't "solo" the central image on a stereo system (not talking about accessing the center frequencies that can be focused on a M/S matrix, but referring to fine-tuning the center image by placement of the left & right speakers), unlike a surround system which does have a central channel.