If you are struggling to get the centre image balanced may I suggest-
.Try swapping left for right speakers and right for left and see if the imbalance is still there.One speaker might be faulty.
.Swap the tubes left for right and right for left and see if that changes anything.That will indicate if there is a faulty/low output tube.
If all that tests OK then you might need to change the balance pot for a different value.Any technician could do that.