I recently faced this problem with my Rega cart. All of a sudden the sound in one speaker was much lower than the other. I couldn't figure it out, I reconnected the cart, checked the wires, etc, but it was still happening. By ruling out every link in the chain, I finally discovered that my phono stage was at fault, and when I plugged the TT directly into my amp's phono input, everything sounded back to normal. So, you never know, just keep checking every link until you can say for certainty that the cart is at fault.