Criss – not much difference really. I’ve been doing electronics long enough to know that there would not be a significant or audible different between the 12nF & the 15nF or between the 15nF & 22nF – but one might be able to hear the difference between the 12nF & 22nF caps. If you notice in the Muff’N Build Document’s BOM section, there are only two different values used for C10 for any of the 10 different build options available – 3n9F or 4n7F; and with such a small variance between those two values, the audible difference is also insignificant. C10 is also partly responsible for the stock BMP’s “mid-scooped” sound. By increasing C10 to 4 times its normal value, that “scoop” threshold is raised, and coupled with a 25K pot allows the operator to flatten and even “bump” that “scoop”.
Once you start building, I recommend that you socket those components in the BOM that have the largest variances between the different builds. This will allow you to quickly swap out component values to see what differences you can hear and assess for yourself what you want your build to sound like. Experimentation and trial & error are the greatest teachers.