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@geek101 Those swaps may obtain better matching of the gain sections, which tend to drift with time. It's also possible that by reseating the tubes you cleared some sludge and restored electrical contact. For a unit that old, you should clean all tube pins and sockets with contact cleaner. Also, consider replacing at least the 6SN7 tubes. Those are in the power supply circuit and tend to wear out faster than the 6922 signal tubes.
The owner’s manual is on BAT’s website.