I've no idea of the impedance, true. And yes there is a filter... the tone control is more akin to a moog-style resonant lowpass filter than a typical passive lowpass.
So turning the knob adjusts the resonant frequency; there's a clearly audible peak at the resonant freq; a fairly steep rolloff above resonant freq; and fairly flat below that. There are useful tones at many positions of the knob, all with ample lows.