Failing or open series dropping resistor in the bypass networks
leading to preamp stages; these are usually 8K to 10K, usually 2W or so.
They form a series string from the highest voltages at the screens of the
power tubes to the plate supply for the input pream tubes. At the junction
of each two of them, there will be a bypass capacitor, usually 10 to 20uF
to ground. These resistors can get hot and drift upwards in value, dropping
the plate supply voltage to the earliest stages too low for proper operation.
They can also be cracked or broken sometimes.
Failing bypass capacitor; those bypass capacitors get leaky, shorted or
open just like power filter caps- test them the same way.