The so called "sweet spot" is the quite small listening location where the two speakers of a stereo system produce a very convincing "phantom" image of a centered soloist. In a multichannel system there is no need for a "phantom" image because there is a real image in the center speaker. And this real image does not move or disappear when the listener moves around the room. Consequently the notion of a "sweet spot" does not apply.
Interestingly when you have been using a center speaker as long as I have the "phantom" effect becomes a bit tiresome, like an old parlor trick. At one time I used Bob Carver's "sonic holography" preamps, and this was like the phantom image trick on steroids! The effect was amazing, but if you moved your head a few inches...poof. Not really conducive to listening to music.
Interestingly when you have been using a center speaker as long as I have the "phantom" effect becomes a bit tiresome, like an old parlor trick. At one time I used Bob Carver's "sonic holography" preamps, and this was like the phantom image trick on steroids! The effect was amazing, but if you moved your head a few inches...poof. Not really conducive to listening to music.