It could be the tubes. try to switch the tubes and if it moves then you can isolate the weak tube.
It could also be the cartridge. Get a tester and put it on 100ohms both channels should have almost the same reading if one channel is drastically way off then the cart's coil is ready to open. before you know it one channel will be dead.
It could also be the cartridge. Get a tester and put it on 100ohms both channels should have almost the same reading if one channel is drastically way off then the cart's coil is ready to open. before you know it one channel will be dead.