The DC balance contols (bias pots) should be on the TOP of the board and the AC balance controls should be on the BOTTOM of the board. The reason is that once set at their mid point the two AC balance controls should be left there unless you have a signal or function generator to REadjust them. They are put on the bottom of the driver board so that you don't accidentally start turning them when you really just want to adjust the DC balance for each tube. Once set at their center position AND you use a matched quad of output tubes, those AC balance controls are, for all intents and purposes, at their optimum setting.
Bob
Bob