If it sounds good at 5.9v that is fine but if you want to further adjust it you will likely have to Mod it since no transistor is equal and some can be really far off now days since they have been majorly cherry picked in the last 15 years leaving a lot more “off” ones than ones that are within a tight spec. (this is also covered in the doc since this is really the case) That said if it sounds great I would not worry. If you want to proceed with modding to get the bias a little lower it will require removing a resistor if the trimmer does not have enough swing to counter the variance in the transistor being used.
We will need photos and voltage readings throughout if you wish to pursue any mods so we can be sure the problem is not tied to anything else because a 20k (trimmer) provides a great deal of variance. That said make sure it is a 20k or Code 203 and you might want to try another transistor if you have one and hopefully it is socketed for ease of testing.
Here is the build doc link for reference: https://guitarpcb.webs.com/PDF%20Files/PUFF_v2.1.pdf