Swap the tubes from the right channel to the left. Does the weak channel follow the tube swap? If so, then one or both of the original left channel tubes are going bad. Swap the left and right channel tubes back to their original positions one by one, and listen for any changes upon each swap. You should be able to narrow down the culprit.
Of course, if the sound does not change after swapping the tubes, then your problem lies elsewhere...
Of course, if the sound does not change after swapping the tubes, then your problem lies elsewhere...

