[pseudohadamard]

You can get hum from ground loops which, given how hard it can be to track down, could end up being blamed on nearby mains wiring because you'll always have some sort of mains wiring in the vicinity of the audio gear in order to power it.

[amluto]

Right. I’ve never heard hum from a loop in the amplified wiring to a passive speaker. I did do the math, and it is certainly possible to arrange to couple in enough power to hear it without amplification, but this isn’t the norm.