The upper and lower lugs on the volume pot on the headphone jack side of the amp need to be temporarily shorted together, then see if the issue goes away.
If the issue is still present, remove that jumper and put it between the upper and lower middle lugs on the volume pot, then recheck.
If that didn't change the issue, remove that jumper and jumper pin 10 on each 12 pin socket together, then recheck.
If that didn't change the issue, remove that jumper and jumper pin 3 on each 12 pin socket together, then recheck.
If that didn't change the issue, remove that jumper and jumper terminal 3 to terminal 17, then recheck.
If that didn't change the issue, remove that jumper and jumper the upper and middle inside headphone jack terminals together, then recheck.
If that didn't change the issue, remove that jumper and jumper the upper and middle outside headphone jack terminals together, then recheck.
Do note that if you can download a 60Hz tone onto your phone and play that through your SEX amp, you can measure AC voltages at a few spots and we can debug your amp that way.
-PB