I like to say what a beautiful system or systems !
I just played Metronome All-Stars 1956 1974 Japanese pressing and did not hear the separation that your are hearing , it is only available in mono so I am assuming the it was recorded in mono . Going with elliottbewcombjr's # 3 that is a possibility , looking at your room your left and right speakers are not equidistant from the side wall so maybe ? Cartridge alignment ? Swap cartridge TT cables to preamp L & R from preamp to amp L & R as your last test .
Have your tried other mono pressings yet ? I have some early 50's jazz that heavily favor the right speaker ( giving the #3 impression ) . If you like the Beatles try the mono pressing of Sgt. Pepper , this is one of the best examples of mono I have ever heard .