btw, some Vintage Amps and Preamps like Fisher 500C had a Center Channel Out which was derived from l+r and powered.
Think original Mono Speaker and Mono Amp in a Space.
Add a second matching speaker and second matching Mono Amp.
Thus ready for Stereo and Big Mono.
Now, stereo placement might not have been possible, and a large space might have a hole in it, thus a derived center out, long before Quad, Long before dedicated center channel content.
Or, drive Stereo Pair in one room, and drive a single mono speaker in another room.
I would check the manual thoroughly, you might have optional use of that Center Channel.