Audio valve preamp

How did you come to the conclusion it is the preamp? Could be something else in your system causing the imbalance?

The first thing to do is swap the interconnect cables at the inputs of the power amps, left for right.

If the problem moves its either the cables or the preamp.

If the preamp stays put is either the amps, speaker cables or speakers.

Thank you for your response , we tried everything swap cables , even set it up with other speakers and amp, same problem ,even changed the tubes.. I was told there are many relays in unit .  hard to fix.