Magnetic distortion is only an issue for high currents/low impedance, as the inductive component of the signal is proportional to the current (ie its completely immaterial for signal processing at low power such as preamps). Once you've got many amps involved at sub-ohm levels the induced voltages start to matter, such as steel cored brass-plated speaker terminals (create measurable distortion at high power).
(Many cheap passive crossovers contain ferromagnetic cored inductors and are just as bad!)
(Many cheap passive crossovers contain ferromagnetic cored inductors and are just as bad!)