Have you tried to switch the valves around? If the issue moves to the other tweeter, then the root cause is with the valves. Are they same brand and model?
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There are two tubes for left and another two for right channel. The two left tubes are new and the same KT88s from Jadis. No idea about tube bias.
Actually I have a separate set of 4 Gold Lion KT88s and the same effect with these as well. So I don't think the valves are the cause.
On further listening, I think the entire left side may be a little weaker. I have tuned the Balance knob over to the left and it seems better. Is it normal to have different amplification levels on the left/right channels? I guess that is what the balance knob is for? I have always had it dead centre before.