WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I agree with the 50 KOhm suggestion. Very few upstream sources find 50 KOhms "unfriendly", while the value is low enough not to adversely interact with the Miller and cable capacitances.