Re: Nikko Preamps, etc.
Yes, Herb is right, that is a common amplifier adjustment as well for 0 DC at the output.
Those pots are for adjusting current from the current source, to balance the long tailed pair, but also correct for other circuit imbalance from downstream stages. The long tailed pair circuit itself should already be pretty balanced, as it uses matched-pair-in-one-die JFETS (2SK68A) and cascode transistors (2SC1775).
This is a really premium preamp phono gain stage design, as it uses both a current source on the tail (versus a resistor) and cascode circuit on top. It may look like a current mirror on first glance, frequently used there, but it's cascode, some feel the best architecture you can have there for audio stages used w/ feedback.
Marsh detailed the cascode circuit advantages in Audio Amateur way back in the old days, with plenty of
example circuits and measurements.
More background here
from the wiki on cascode
"The major advantage of this circuit arrangement ... the upper transistor permits the lower FET to operate with minimal negative (Miller) feedback, improving its bandwidth."