MOSFETs have a rather narrow window where they work as resistors, and the exact threshold voltages vary between transistors (The Vgs(th) for STP16NF06L is specified as 1−2.5 V).
Sam’s design assumes it’s at least similar for all MOSFETs on the same board (e.g. that they’re from the same batch), but you will need to tweak the RV24 trimmer to make sure the internal CV is in range. Easiest is to try a few trimmer positions, tweaking the master tune at each position, until you find a setting where the master tune has the most impact.