The COB is an interesting circuit, albeit a bit of a misnomer. The octave itself is not "clean". However, the Blend control allows one to combine a clean signal with differing amounts of octave fuzz, where most other octave-up units confine one to either octave-doubling all the time, or octave/no-octave.
Here's a mod that might be worth exploring. The "clean" signal is full bandwidth, which means that it has all the harmonics of the original signal. The octave can be made to sound more obvious by filtering out the highs from the clean signal. So I will suggest the following:
1) in the "clean" path leading up to the clean side of the 250k Blend pot, insert a 10k and 4k7 fixed resistor in series, with the 4k7 tied to the clean side of the blend pot. This will reduce the maximum amplitude of the clean signal, but not so uo'd notice. After all, a 10% deviation from niminal value on the Blend pot is 25k, and what I'm proposing is only 14.7k.
2) Run a wire from the junction of the 10k and 4k7 resistors to the common of a SPDT on-off-on 3-position toggle switch.
3) To the ouside lugs of the toggle, connect a .01uf and .027uf cap, whose "free" leads are connected to ground. When the toggle is set to the middle position, the caps have no effect. When the toggle is set to the side positions, you'll have treble rolloffs on the clean signal alone, that begin around 1.6khz, and around 590hz, respectively, for the .01uf and .027uf caps. This will dull the clean signal so that the octave side will be contributing the harmonic content. This will not only allow the octave to be a little more obvious, but will also mean the Blend control can act a bit like a treble control. It also means you can introduce some clean bass into the octaved sound without having the treble content of the clean side obscure the octave-fuzz sound.
Because this is simply a 6db/oct filter, the treble rolloff will be gentle/modest, even though the corner frequencies seem low, and some clean treble will still come through, just not nearly as much. Raising the value of either cap will shift the rolloff point lower, and making the values smaller will move the rolloff point upwards. The nice thing is that the filter is easily defeated with the toggle.
One thing that is missing here is the diode clipping pair near the output found on a number of other much-liked octave units. As I noted in an earlier post, those diodes form a crude compression circuit that allows for the output signal level to be held fairly constant, such that the doubling of the fundamental seems to bloom as the note is held. Without that simple "clamping" of the output level, the octave may seem to fade out very soon after picking. Never having played with a COB or the COB circuit, I can't say if that would improve or degrade the normal sound of it.
FInally, consider swapping the silicon diodes in the circuit for Schottky type. BAT4x or 1N5817 should do just fine. That will make it easier to get octave doubling on lower frets, not just the stuff above the 7th.