Not sure if this problem is on the vertical or horizontal amp, but the trouble-shooting should be similar.
First, the balance pot: on early s/n's this was 5k, changed to 10k on s/n 6630 up . 100k will reduce the DC's on the screen grids.
Most likely candiate for imbalance is the 12AU6 valves, so you could try swapping over the relevant pair and see if the imbalance is similar but in the opposite direction. Ditto with the following 6AU6 tubes.
If it's not the valves, there are a host of resistor pairs to check: you're looking for symmetry across the two halves of the differential amps - the DC voltages measured with the balance (and differential) pots centred should provide a clue.
John
John Sykes