It would be helpful to know if this is "new" hum that wasn't there before. If that's the case, then this should be a rather easy problem to find, as the phono preamp section by itself is not prone to produce hum in the 400C. If it's "old" hum that still hasn't been killed yet, then other issues may still be present.
Assuming it is new hum, first verify that it is absolutely from the phono preamp section by selecting the low level preamps, but reducing the individual level controls for this section to zero. If the hum is gone, then it is in fact being generated by the phono preamps. If not, then selector switch wiring should be examined.
If it is from the phono preamps, there is little that can cause hum in the actual preamp module itself, with only one audio ground in the module, and heavy shielding for most of the components. Therefore, I would first inspect the DC heater supply to make sure that a good ground reference still exists for this circuit.
In the original design, this was provided by the fact that this supply had its negative side connected directly to ground. However, if you installed the bias network to raise this supply by about 65 volts above ground, then the 10 uF 100 volt cap in the new network providing this bias voltage now provides the ground reference for this supply. Therefore, if the cap is defective, or improperly connected, then the DC heater supply will not be properly referenced ground, and the phono preamps will likely produce hum in that scenario.
Properly installed, any direct connection between the negative side of the DC heater supply and ground from the original design should be removed, and the output of the new heater bias network connected to the negative side of the heater supply. In the output of the heater bias network is the above mentioned 10 uF cap, with the positive side of this cap connected to the negative side of the heater supply, and the negative side of this cap grounded. So connected, this cap then maintains a reference to ground for the DC heater supply to minimize noise.
Check to make sure that the negative side of the DC heater supply is in fact biased by about 65 volts above ground, and that the cap mentioned is correctly installed. If not, that is the likely problem, and if so, it eliminates this possibility before proceeding on.
Dave