Wahs boost, and they distort if the input level is too high. But more gain gives more effect.So a gain control is good for matching the pedal to the guitar. A little distortion adds color, but I don't like buzzy crackly distortion. Some transistors distort more horribly than others.

You can socket a transistor as shown below and try different kinds. 2N3904, 2N5088 or whatever NPN transistors you have lying around. Be sure you get the pinout right. I seem to end up liking MPSA18, gain about 550.

Q1 has a 22k collector resistor and a 390 or 470  ohm (it varies, a few hundred ohms anyway) emitter resistor. You could replace the 390 ohm resistor with a 1K trimpot if you like. Reducing the resistor increases gain. Or just use a lower value of resistor for more gain, a higher value for less gain and distortion.

The 1k trimpot would do but it's time to Ramble On.:

Reducing the emitter resistor increases gain and reduces the DC collector bias. Reducing the bias increases the chance of unpleasant distortion, I think.

The other possibility is to use a 820 ohm resistor (or whatever gives you 4-4.5V on the collector) and bypass it with a (100uF cap and a 1K trimpot in series). That has no effect on DC bias of the Q1 collector, so you might get more gain without more distortion.

I used a 10k log pot, I might mount that on the outside. I chose a transistor that distorted gracefully.