I think the thing to be clear about here is that tubes from a matched set that have been used for any length of time may not be a match anymore! Without measuring their individual cathode currents in circuit you can't know.
I had a similar problem with a older marshall 50 watt head after redoing the PI and bias supply I still had the same issue. The fix,and I don't know why was replacing all the high voltage power supply caps, everything was fine after that, I guess one must have been shorting out.
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