I’ve only demo’d Mason FuSang briefly and I enjoyed its mids very much. The problem I had was the bass and treble.I like Mason FS better for Mids than Mentor.
There’s been quite a few midrange specialists that I’ve owned or heard in the past that are on par or even exceed Mentors mids such as Phonix/Erlkonig/Elysium and even the Rhapsodio Supreme Mk2, but all of them suffered in either the bass or treble or both. Mentor to my ears has the least sacrifices across the FR, so that’s why it would be my choice. Ymmv