There's a site called DIYAudioHeaven that does provide schematics for analog filters for certain headphones. And I've seen speculation that the Dan Clark Audio Expanse uses a passive filter to produce its bass shelf that would normally be impossible on an open-back planar-magnetic headphone. So it's not unheard of to use passive filters, but certainly not commonplace. I suppose people who use high-output-impedance amps on headphones with highly variable impedance curves are doing some passive filtering too.