This mod is verified and is quite effective. It replaces the 15k resistor in parallel with the .01uf cap. Lowering the resistance will give you a darker tone. I found that 6.8k was the cut off before distortion. So if you notice any distortion at full turn counterclockwise, you should use a larger limiting resistor, like a 7.5k or 8.2k. Increasing resistance will give a brighter tone, but extrapolating 100k doesn't do much. A log taper pot is definitely the best taper to use here. A linear taper works ok here as well if you don't have a log taper, but there will be "crowding" in the darker rotation of the knob and not much going on in the brighter rotation of the knob.