kel84 channel imbalance

[chambers67]

This is my first post and concerns my KEL84. There is a discernable channel imbalance, with the right being noticeably quieter than the left. The valves have about 100 hours on them. Swapping the valves from left to right has no effect. The problem remains when I use my Revolver speakers or Heybrooks. My knowledge of electronic circuitry is sparse to say the least, but any suggestions, in abc form, would be most welcome. I can use a soldering iron, although my technique is not perfect.
I purchased the amp from Ebay, to be used in another room. I also have a Pure Sound A10, which performs faultlessly, making the KEL fault even more frustrating.