From the FAQ
NAC282:
Upgrade 1 socket powers the left channel: 729 input filter stage and tape buffer off one regulator. This regulator also powers one side of the aux2 output (for the right channel of a stageline), gain stage off the other regulator
Upgrade 2 socket powers the right channel: 729 input filter stage and tape buffer off one regulator. This regulator also powers the other side of the aux2 output (for the left channel of a stageline), gain stage off the other regulator