Mono record sounds like stereo

@adrianleewelch ,

You might want to check out the recordings from pre-war 1930's Germany.

I have always been amazed at how much sound they could get out of those microphones.

Bob

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 .

Good Luck

Oh dang. It never occurred to me to swap the RCAs as a quick test. I’m going to try that now. TBH I’m not even sure if my Preamp has a mono setting. I’m going to check that as well.    Thank you !

Get a 12" with a removable head-shell so you can easily swap cartridges. I'm not sure it matters which arm hosts the mono. Try both and report back. I have 12" and 9" arms on my main table. 

To be more specific not the whole cable but just the input to each component ,

if anything unplugging and plugging help clean the connections.