you can always make a passive volume control with a pot by making a voltage divider. there are a couple ways you could implement this. I would probably put a pot in series with the output of the effect and use the wiper as the output (with the third lug going to ground). you don't get a boost with this configuration, however.
I wouldn't use R18, although fundamentally it would probably work. the difference is the fact that C7 and R18 forms a high pass filter, so changing the resistance changes the corner frequency of the filter (changing frequency response at different gain is usually the sign of poor design).
if you're up for doing some testing, R17 seems to be the feedback resistor for the IC1B. if you make it bigger, the gain of the circuit will increase (thus more volume). if you replace it with a pot, you could have an output gain control. there is a problem, because R17 also forms a filter with C6, so the frequency response of the circuit will change somewhat. I would try something like reducing R17 to something like maybe 30k and putting a 50k log pot in series. this way, the resistance doesn't change _too_ much but probably enough to give you a little gain boost (or drop).
I wouldn't use R18, although fundamentally it would probably work. the difference is the fact that C7 and R18 forms a high pass filter, so changing the resistance changes the corner frequency of the filter (changing frequency response at different gain is usually the sign of poor design).
if you're up for doing some testing, R17 seems to be the feedback resistor for the IC1B. if you make it bigger, the gain of the circuit will increase (thus more volume). if you replace it with a pot, you could have an output gain control. there is a problem, because R17 also forms a filter with C6, so the frequency response of the circuit will change somewhat. I would try something like reducing R17 to something like maybe 30k and putting a 50k log pot in series. this way, the resistance doesn't change _too_ much but probably enough to give you a little gain boost (or drop).