Designing parallel RLC band-stop filter in series with load from f0, gain, Q parameters
In general, designing passive filters to work at the low, highly frequency dependent (i.e., reactive) and poorly-controlled impedances associated with things like headphones is not the best approach.

It would be better to attenuate the headphone signal as needed to run it into an active (opamp-based) filter with the desired characteristics, and then boost it back up to headphone levels at the output. The results will be much more stable and easier to fine-tune for specific applications.

In fact, you can purchase such units &mdash; both parametric and graphic headphone equalizers &mdash; off-the-shelf.