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KA-8100 Humm (60hz)

It reads 0.02 ohm. That is one thing am unsure of. It looks like it has been re-soldered when I look at it, and I have a feeling it's been soldered to the wrong place. There are 3 grounds that meet on the pre-amp:

The signal ground comes from pin 5-6-7 connector (pin 6 is ground), and travels all the way through the preamp. It connects to the volume pot's actual volume pins.

The power ground, comes from the rectifier diodes and big capacitors of the amp, and connects to B+/B-/Gnd on top of the preamp board. It feeds into a power regulator Qi13/Qi14 (which I replaced because all that stuff around that area was burned with overheating). It is decoupled from the signal ground via diodes and capacitors.

The chassis ground, comes from the nearby chassis ground point, and is connected to the volume pot's chassis along with the other pots' chassis.

Following those 3 grounds, they look like they are no creating any ground loop, but clearly there is one at play.
 
Only when I unscrew the bottom case, then touching the volume pot makes a hum. Otherwise, when the case is screwed in, no.
 
But if I reached out way into the amp and touch the signal wires to the volume pot (on their plastic), I hear a loud hum.
 
One more detail, I have a KA-6100 as my main amp, and it is absolutely silent no hum or anything. I can plug it in the same outlet and still silent. The 8100 definitely has an issue that I can't put my finger on.
 
Wrapped the signal cable from the pre-amp board to the control board with copper foil tape, then with electric tape (to make a shielded cable out of it) and soldered it to the signal ground (that cable already has 3 such ground wires) and that eliminated 90% of the hum and radio interference. Now when I crank it all the way, the radio and hum are barely audible. They're still there, but very very very faint.
 
Yeah, if i remember right, signal cable runs into a channel in the chassis, from back to front, shielding it.
 
Replaced the volume/balance pot with a bunch of resistors (50K and 100K) following the schematic to fix it to 50% volume. Plugged my laptop in AUX input, and it is dead silent (and plays very well). Problem definitely is around the volume pot.

I took it apart, and the little metal brushes seem very damaged, and the resistive surface quite scratched. Looks like there is poor contact inside the pot, which causes the issues.
 
All right! Finally fixed that hum!!! Turns out the problem was with the volume pot, everything else (adding extra shielding to the signal cable, my friend changing the transformers) reduced but didn't fix the hum.

Conclusion:
- There was a 120Hz hum created by Qi13 and Qi14 having gone bad (excessive heat in that area of the amplifier). Changed them along with the surrounding capacitors, diodes and resitors.
- Volume pot was cracked (pcb and little metal brush), couldn't fix the original, so I replaced it with an ALPS 100KAX2 100K Ohm. Was able to slide the original balance pots around the shaft too, after some jerry-rigging. Looks original from the outside, and plays nice!!

Thank you @turboyam and @Semanon for the help!!
 
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