That's an idea : Filaments are dc fooded but cathodes aren't decoupledIn my experience, an AC filament supply works fine in an MM amplifier when you use an EF86 in the first stage, decouple its cathode to ground with a big capacitor and use twisted and shielded wire for the filament connections.
Edit: apparently I'm repeating myself, see post #12, https://www.diyaudio.com/community/...-regulated-stabilized-psu.374452/post-6716476
regards