Apparently it does not have an overall master volume. As I am not intimately familiar with all the settings and features I found this from a discussion somewhere else.
There is no master volume control as the HX Effects is essentially a unity gain device. Meaning that with everything bypassed there shouldn't be any gain or attenuation. You want to make sure you have the inputs and outputs set at the correct level, though. If you have it in front of an amp, you'd want them both set to instrument level. In a loop, you may want line level. It depends on the amp.