Niagra 7000 HUMM

How much niagra 7000 humm is prevalent and does it bother anyone or interfere the quiet passages in music 

Ag insider logo xs@2xrizwanalimohd
Do you own one that humms? If not why would you expect that it humms?
i am about to purchase one used possibly
i have one and minimal hum after following AQ’s instructions. 
I used to manufacture power conditioners with isolation transformers- Silver Circle Audio. TCHAIK 6. It had a 70 pound toroidal transformer. Hum - is caused by DC offset in excess of 1 volt. The manufacturer should be able to direct you to a circuit that removes the DC. I used to build them when needed. Hope this helps. 
what are the usually culprits?