The input stage is not the source of the dc. Since the offset is worst
with the pot in the middle this indicates the leakage is from the
amplification stage immediately following the pot's wiper. I'll bet
there is a coupling capacitor inbetween which is leaky.

With a 28 year old piece of circuitry it is possible to have a leaky
electrolytic capacitor here and there, especially if it sat idle for
some months, years, decades, and they may just need "reforming".

If the amplifier is truly DC from input to output then the problem could
be the offset trim or balance is off. Perhaps a resistor went up in
value or the trimmer has a dirty or noisy contact. It may also be a
leaky transistor though.

The amplifier is truly DC.
There is a switch just after the potentiometer. It says "DC or Low cut".
If the switch is in "Low cut" the signal goes from the potentiometer through
a capacitor to the input of the amplifier. If the switch is in DC, the
signal goes directly from the potentiometer to the amplifier.
If the switch is in "Low cut" the amplifier goes in protection mode, because
then the potentiometer can no longer influence on the DC level.
I have now replaced all 5 electrolytic capacitors in that channel, except
for those in the power supply just after the bridge. They are a bit special,
and their physical form needs to match for an easy replacement. I have
checked some additional transistors in the middle section of the amplifier
they were also fine.
Since I have DC out on the input, I expect the error to be in the very first
stage of the amplifier. Don't know if an error further back, can cause that.

/Tom