I have owned the Furutech DeMag since 2009. One of the best tweaks I have purchased over the years for my analog system. The carbon that is added to the vinyl formula to produce black vinyl is where the record gets the magnetic property from. It is a small amount but enough to effect playback. The stylus is very sensitive to things like this regardless of who the cartridge manufacture is.
I watched an interview with Michael Hobson "owner of Classic Records" at the time, about this very subject. He told the story about how they were listening to some vinyl albums at a friend’s house where they were at did an A-B test with one of the albums they were listening to using the DeMag. The friend did not tell the group what he was going to do. Listening to the album after the DeMag process Michael Hobson asked after the first track played, "What just happened here". This sounds amazing! This is where the discussion started about how carbon effects the sound of a black vinyl record. And this is where Michael Hobson got the idea for Clarity Vinyl. Mr.Hobson and his people worked on removing the carbon from the vinyl formula and that is how Clarity Vinyl came to be. It is considered today as one of the best vinyl formulas in the industry today, no magnetic properties at all.
I know it is a very controversial subject and I am not here to convince anyone but suggest you give it a try, the results may surprise you.