Hello

> The range of the tuning knob can be modified by lowering the resistor R5
> (1M). If you replace the 1M e.g. by 100k you obtain about a four octaves
> range (but then it's more difficult to tune the VCOs).

I recommend to solder another 1M resistor in parallel to the existing
R5. This increases the range to a little bit more than one octave, but
the range is still small enough, to do a satisfying fine tuning.
This is the most easy thing, as you do not have to desolder the existing
resistor.