K4301 pink noise generator

Can the filter be extended with R4/C4 R8/C8 and R9/C9 (it is shown on the schematic diagram but not in the parts list) and if so what are the resistance and capacitor values needed?
In the manual it states that the lower frequencies are unstable, what does that mean? That the device can’t be used for those frequencies?

Normally if a circuit is unstable it should not be used at those frequencies.

As this is one of the very early Velleman kits, it is hard to find documentation regarding the design.
The people that developed this kit are no longer with this company, so we cannot ask them.
We will however dive into our archives and see if we can come up with something helpful.