That seems understandable.
When the preamp's mute function is activated, it forces a zero-voltage condition on its outputs, apparently by direct mechanical action of the switch since you indicated that the switch has that effect even if pressed while the preamp is off.
When the mute function is off, and the preamp's power is off, the un-powered output stage of the preamp is connected to the power amp instead. That would basically be a high impedance, that would exert very little control over what goes into the power amp, the result being increased susceptibility to hum pickup.
The hum may result from ground offsets between the preamp and power amp, or from emi being picked up by the interconnect, or other means.
Are the preamp and power amp both plugged into the same ac source (power strip, line conditioner, outlet, etc). If not, doing so might reduce the hum, by reducing possible ground offsets between the two components. But I think that basically it's a non-problem, since you can eliminate it by simply muting the preamp, and its cause is not anything that might be harmful in any way.
Regards,
-- Al
