Well the new tubes have been playing for 7 hours and one channel lower the hum considerably. I hope as time goes by the tubes will stop making noise.
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Could be due to a greater amount of heater-to-cathode leakage in the new tubes, interacting with whatever amount of AC ripple may be present on the voltage that is provided by the preamp to the tube filaments/heaters. I once experienced that issue, with different equipment.
If you have or know someone who has a good tube tester, such as a vintage Hickok, you could measure that leakage and see how much it differs between the various tubes.
If that is the cause of the problem, as far as I know it could continue to improve, or stay the same, or get worse over time, with little or no predictability.
Also, it might be worthwhile interchanging the two new tubes between the two positions in the preamp, unless you have reason to believe that the circuit applications in which the two tubes are used are identical.