120hz hum coming from all-tube system - but only at high volumes

I generally listen to my system at 8-9olcock on the volume dial.  A friend and I were pushing the dial up to 11-12 oclock and a fairly loud 120hz hum would start to appear after a while.

Bringing the dial back down would eliminate it.  You could then pushing it back up and it'd be fine for a bit and then then hum would reappear.

Do I have bad tubes somewhere?  Ground loop that only rears its head occasionally at high volumes?


(System is Manley Steelhead pre, Manley Mahi's mono amps, Forman Elite-15 PFi power conditioner, schiit yggdrasil DAC, Klipsch Forte IV speakers)