Looks like Lars Lundahl (famous for high quality audio transformers) wanted to build an audio magnetic amplifier in the 90s, but the project got canned. Some German company ended up making it a decade or so later

http://www.auditorium-23.de/MagAmp/MagAmp.html

http://www.acousticplan.de/html/magamp_english.html

The technology has also been used in a completely passive multi-band eq/saturator - https://www.soundonsound.com/reviews/gyraf-audio-gyratec-xxi...