I agree, even though the OP said in post #9 "I've measured each pot resistance evolution while turning variable resistance and they behaves normally" even so, definitely try another pot.
That is a mystery. I'd think there was an incorrect resistor somewhere but it's only VREF going into the 301 and you verified that your VREF is correct.
I also thought maybe you've got a bad/fake TL072, but you're getting the same reading at pin 3 of the 301 IC... I don't know how/why, but it's gotta be VREF-related?
MAIN PROBLEM:
c.est.baastien, Seems there's a short with the volume control if it's basically maxed no matter what the other controls are set at.
With a DMM set to continuity, poke around the VOLUME pot and nearby components to see if continuity-beeps exist where there should be none.
If you find such unwanted continuity, then you can explore further visually to see if there's a strand of wire, or spatter of solder that is making the unwanted connection.
Continuity between VOLUME PAD-3 and PAD-2 on the PCB?
En aparté ...
I would suggest standardizing your wiring.
GAIN & VOL are wired: LUG 1 BLUE LUG 2 GREEN
LUG 3 BLACK
...HI & LO are wired: LUG 1 BLUE
LUG 2 BLACK LUG 3 GREEN
Since you're using black on your power ground, you could wire the VOL with black on Lug-1, as it goes to ground, but not all pots' LUG-1 go to ground...
Nonetheless, I'd keep the wiring consistent on all pots (*and since I only see four colours in your build — black blue green and red reserved for power):
LUG 1 BLACK (often goes to ground, or combined with LUG 2 in a rheostat-wired pot)
LUG 2 BLUE (usually the OUTPUT of a pot when wired as a voltage-divider as opposed to pot wired as rheostat)
LUG 3 GREEN (usually the INPUT of a pot, whether voltage-divider or rheostat)
Sometimes the desired rotation of a pot calls for the in/out of LUG 1 & LUG 3 to be swapped, but even then I keep my wiring consistent:
LUG 1 — colour A LUG 2 — colour B LUG 3 — colour C
Not necessarily those specific colours, but the point is they're consistent within each build. I generally use black for ground and of course red for power, then I have a few other colours of wire for in/out of jacks, 3 different colours for pots as mentioned...
Makes trouble-shooting much easier. Maybe buy one more colour of wire.
Of course, do whatever makes the most sense to you; some people use one colour or wire for their whole build, on all their builds.
En aparté deux...
There looks to be a stray strand from the power wire to the anode of the LED.
