Discussion about the Korg PolySix synthesizer

Most other Korg equipment from roughly this timeframe expects control
voltages between -5V and +5V for filter control as well.

Negative voltages probably lower the cutoff. Now, depending on the
functionality of the LFO in the modular system you may have an LFO that
ranges from - to + or one that ranges from 0 to + (your description sounds
more like the former).

When the long LFO times create silence it probably means that they lower
the cutoff frequency too much. I would start by turning up the cutoff more
and by attenuating the external LFO.