Just for purposes of diagnostics, can you run it off batteries for a bit? This will entirely eliminate PSU related ground loops as an explanation; I would have though +/- 9V for the input stage and +/- 18V for EQ stage would work well enough to hear if the hum is still there. Nasty little PP9, tranny radio batteries would do for the test.
If the hum is still there, it isn't related to PSUs or PSU earthing, and must be getting in somewhere else.
If the hum goes away, it is either coming in via PSU, or is related to PSU earthing.