Palewin, When the preamp is off it has a high impedance- so its nearly the same thing as having the interconnect cables connected to the amp with nothing on the other end.
When the preamp is on, its output impedance is low and you don't get hum pickup.
IOW, the hum is absolutely, completely normal- don't worry about it.
When the preamp is on, its output impedance is low and you don't get hum pickup.
IOW, the hum is absolutely, completely normal- don't worry about it.