Wahs usually cut volume slightly, not good for the big Wah solo. Reducing the 68k resistor to 22k gives a decent volume boost, but obviously increases distortion a bit. Probably only good for the 3 transistor wah with the input buffer.

You can't put a volume control on the output of a Wah. The Wah pot is already there. You need a high impedance buffer circuit before the volume control, same as the Darlington or FET input buffer on the 3-transistor version. That's easy to add, 2 resistors, a capacitor and a JFET. You could use a FET volume booster like the Alembic Stratoblaster there. Too much boost would probably distort, so maybe  6db (double original volume) would be enough. You can have a volume control after that, an internal trimpot or an external knob.

I have a PCB for this, it has an input FET buffer in case you don't already have one, and an FET volume booster with the internal trimpot. You could use an external volume knob.