Mono record sounds like stereo

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. 

adrianleewelch OP

Definitely, without doubt, put the stereo cartridge on the long arm. Pick an arm with removable headshell if possible, gives you flexibility now and in the future.

Does your 9" have a removable headshell?

My 12.5" long arm is fixed cartridge, used for my MC cartridge. Only change it when worn.

My 9" has both removable headshell and extremely easy arm height adjustment, so it is most flexible. I  have several cartridges I alternate, MC, MM, and now an alternate Mono. And we play my friend's cartridges on it. SUT handles various MC impedances, and has Pass for MM.

I added a 3rd arm for mono, fixed. Limiting, but my 9" arm saves the day regarding flexibility.

My Grado Mono is elliptical, on the 3rd arm. I'm very happy with it. I'm not looking for perfection, or even 'best', just gaining the advantages of playing mono lps with a true mono cartridge.

https://gradolabs.com/cartridges/prestige-series-specialty-cartridges/item/79-mono-phono

I just had VAS put a new boron/shibata on an AT33PTG Mono Body. For modern Mono, Haven't used it yet. He played it at his shop, we all were impressed, no comparison here yet.

3 arms: MC, MM, Mono all ready to go in seconds in any listening session.

The compliance of the cartridge and the mass of the tonearm must ’avoid a problem’. https://www.ortofon.com/support/support-hifi/resonance-frequency/

Today’s equipment, I would simply put my existing stereo cartridge (likely high compliance) on my new long arm (likely not too heavy) and expect no problem.

Perhaps, before you buy it, pick an arm you want, tell the members here about it and your cartridge, get their help.