Good work! May I suggest a schematic for bias pots? Pots are 100k/lin, (50% on the scheme just means that program needs % of "opening" the pot...); in the middle position we have about -65V out. Resistors R2 & R3 (between the wiper and "input" leg of the pot) are not necessary, but it`s a precaution if wiper gets discontinued. The useful range is about -50...-80V.