To move just one notch, the tuning of the pole-pair in the corresponding section is all that needs to be changed. Note that tuning affects only one coefficient in the second-order allpass structure. (Although the coefficient appears twice in the transfer function, it only needs to be used once per sample in a slightly modified direct-form implementation .)
The depth of the notches can be varied together by changing the gain of the feedforward path.
The bandwidth of individual notches is mostly controlled by the distance of the associated pole-pair from the unit circle. So to widen the notch associated with a particular allpass section, one may increase the ``damping'' of that section.
Finally, since the gain of the allpass string is unity (by definition of allpass filters), the gain of the entire structure is strictly bounded between 0 and 2. This property allows arbitrary notch controls to be applied without fear of the overall gain becoming ill-behaved.
Phaser Notch Distribution
Classic Virtual Analog Phase Shifters