Why don’t you contact guy directly in Australia. I used to have schematic but I can not find it . Mod is very simple – you connect AU90 instead of toroidal transformer and take out entire crossover network – as I remember. So no caps and resistors in backplate. AU90 has natural frequency slope with bass transformer.
Will – who was real SL enthusiast and genius used his SL with wide open tone controls on backplate – I do the same .