Right, should have clarified this. The equivalent circuit of course accounts for “everything” including mechanical elements because they are attached to the crossover. Again however, equivalent passive circuits assume an inherently linear system, because the circuit is made up of ideal passive components. Is this valid? Yes, for most speakers, because when a speaker’s audio quality is checked at levels below its rating, the distortion is usually well below -10 dB, which means nonlinearities contribute very little to the behavior and can be ignored for purposes of modeling. if one wants to model the effects of distortion at high power handling, then the model has to include elements which distort, I.e., nonlinear elements. As an example, a variable capacitor (varactor, varicap, etc) whose capacitance varies with applied voltage is a nonlinear element. These are useful for modeling power-based nonlinearities in an otherwise linear system. Same idea for inductors - once their cores saturate, they can be modeled using similar nonlinear techniques.