Does anyone know how the low tone control of the Boss Metalzone works?
According to the Wikipedia article, the simulated inductance has a value of

$$L = R_{012} R_{013} C_{009} = 10.34\text{ H}$$

This inductance is in series with a resistance equal to R012, giving an L/R time constant of 4.7 ms, which corresponds to a frequency of 33.86 Hz. This is the lower frequency limit for the bass control's effect.

Similarly, the series combination of C008 and R012 has a time constant that corresponds to a frequency of 328.8 Hz, and this defines the upper limit of the  bass control's effect, and corresponds directly to the inflection point shown on your graph.

Note also that the series combination of C008, R012 and the simulated inductor is also in series with a minimum source resistance of 22 k&Omega; (either R014 or R015), which insures that the circuit is overdamped (no resonance). If this source impedance were reduced, you would start to see a resonant peak at around 105 Hz.