I could still hear the Hum.
I entered the box at about 1:40 am last night and and soon as I lay down quietly inside it, the Hum was there, as loud as ever.
As a final step, I need to establish that the box does what it says it does (i.e. block essentially all VLF radio signals), and to have a physicist confirm my calculations regarding the radio skin depth of a 10 kHz radio wave for mild steel. If everything checks out, then this a serious piece of evidence against the VLF theory.
I’ll let you know if and when I’ll be shifting gears and moving on to the second hypothesis.