Swap the leads over from arm to phono stage. If the down channel moves to the other speaker, then you know it's the leads or the cart at fault. If the down channel stays the same, the fault is downstream. If so, do the same with the leads from phono stage to amp.
“Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”
Hunter S Thompson