Place your multimeter pins on the points silkscreened as TP2 and TP3 around the IFT101 transformer ; the reading should be through the roof, as was mine shown here (0,657V).

Adjust with a non-magnetic whatever the recessed red pot located on the right side of the box (don't touch the other one) : use plastic or wood and nothing else.
Difficult to access, turn, touchy, touchy, until you get something that is as close to 0 as possible ; plus or minus don't matter.
Once done, stereo in all its splendor and if the bass player builds to a frenzy a fiesta for low Es on his 6-string... it won't matter : no more muting.

(that you Pat66 !)