loud enough to hear at the listening position, but just barely. Quite noticeable when standing at the rack.
I have about this same amount of noise, at least. Every time I think it might be worth trying to eliminate it I think how much time and effort that can eat up, and for how little, and balance that against how likely it is I am even able to find it, and say naw not worth it. Then I read one of these stories and say to myself, good man miller, pick your battles. The improvements made the day I spent tweaking my turntable are monster, and instead could have had naught but frustration to show had I spent that time tracking down noise instead.
Then I drop the needle and whatever the volume even extremely quiet vinyl is more groove noise than the other, so what really would be the point?
Just sayin'.
Anyway, the one fact that stands out to me is
Hums with any source, not volume dependent, still hums with no source
components attached (I even tried unplugged them from the wall too). But
the hum stops if preamp is disconnected from amps.
To me the simple fact the volume does not change tells me this hum is not coming FROM the pre-amp, because then it should increase when the pre-amp volume is turned up. Only thing I can think of is its an interaction WITH the pre-amp.
Since you have already tracked down and eliminated everything there then the only thing left seems likely to involve a rather detailed technical knowledge of your amplifier. And with that I can only punt and hope Ralph is there to make the catch and run out the play for a touchdown.