The cap is on the input. cut the jumper and you are ac coupled instead of DC coupled if you have offset from your preamp. The control is AC balance for the drivers to be adjusted with a distortion analyzer.
Unless you have the following test equipment; a scope, signal generator, dummy load and distortion analyzer. You should not adjust the AC balance. If the 12AT7 tubes along with the output EL34's are replaced, then you would adjust for minimum distortion at 1 watt output 1kHz into the dummy load. The AC balance compensates for small phase gain differences in the tubes. There is normally not much shift in the adjustment when replacing the outputs alone. If there is a lot of gain difference between the sections of the driver tubes then adjustment may be required.
Any competent audio repair shop should be able to adjust the AC balance for a fee. The more you want them to do, the more it will cost. So if you really want to have it checked after a complete retube that you have had running a couple of weeks to burn-in the tubes, make an appointment for a shop to do the adjustment while you wait. Be advised, there have been a couple of instances that the AC balance control has required replacement after adjustment attempts. This was on units over 7 years old.