hum out of speaker indicative of ground loop
hum out of box indicative of DC offset causing toroidal vibration
hums are annoying. You might try a DC filter ...ala,
https://avahifi.com/collections/accessories/products/humdinger-dc-line-blocker
Hegel hum?
Spent a week or two enjoying my new H400 and noticed a faint hum between the time I turn it on and before I play music. Also there when I pause after playing a while. Note that the hum is only apparent when I bring my ear a foot away from the amp. I’m quite pleased otherwise but am wondering if this is normal?