The jack that switched the speakers off in the SEX amp was only in production kits for a short while. We can't get a suitable jack at this time. The current jack still cuts the speakers, but no longer has the 120 ohm resistance to equalize the level across different impedance headphones.
The impedance switch board has three outputs - Pos, Neg, and Ground. The tip-ring-shield jack goes to Pos and Ground, the balanced outs get their signal from Pos and Neg.
Yes it took me some time to figure all that out!