Hum could be caused by the heaters being fed AC (wires aren't twisted either) or could be picking up hum on the anode of input tube as it sticks out above the amp and is not screened.
In the latter case getting near the tube with your hand should change hum.
If that isn't the case it most likely is the AC on the heaters.
Also possible that it is the wiring layout but when changing tubes changes the hum this may not be the case.
With good design, AC on the heaters won't cause a hum. If anyone knows what the heater power circuits look like, there may be an easy fix, which will also get rid of the sensitivity to tube choice. Generally, the heater power should be run balanced with the circuit floated up to a DC potential above the cathode, and everything referenced to ground at AC.
Here is a 6SN7 preamp that uses AC on the heaters and is dead quiet irrespective of the 6SN7 chosen, and is configured the way I just described. R11, R112, and C109 do the DC biasing and AC ground reference. And note the way the heater winding is configured.
The very worst way to run heaters is to not have the heater supply tightly ground referenced. The second worst way is to reference one side of the heater supply to ground. In either of these cases, heater-to-cathode leakage can cause a hum problem. And this is a VERY common design error, especially from cut and paste "designers."
If the heater winding does not have a center tap, you can create one by connecting two 100 ohm resistors in series across the winding, then using the junction of the two resistors as the artificial center tap.