An easy check would be to take the Shure/headshell from the Technics and install it on the Marantz (I assume it’s arm has a removable headshell) and see if the issue is the same. At least that would narrow the problem to the Shure/Jico, or not.
So we switched the headshell/cart over to the Marantz and did not experience this imbalance of channels. Afterwards, I moved the headshell/cart back over to my TT to compare. She could not hear what I was talking about until I played a song off an album I have on both vinyl and CD in a back to back comparison. After this, I tried switching the turntable's L and R RCAs on the amp, and the quiet side was still quiet, just coming out of a different speaker. At this point, I'm thinking its either the wires in my tonearm, or the RCA cables on my TT.