It turned out the real issue was with capacitor 128... it was leaking and dragging down the voltage at that voltage divider. This caused the KCV (filter control voltage) to come out at 3 volts instead of 5, and caused distortion coming through the filters.
It is rare to see a ceramic capacitor fail, but clearly with an older instrument like this components that are usually reliable can ultimately go bad.
|In red are the OP amp VCC supply for the filter DAC|
|Here is where it is located on the actual PCB.|