MPV + Stereo downmix

I'm loving MPV for HDR10 encodes in 1080p.
Keeping it 1080p, shows a lot less banding than going the 8bit path.

It's great. It may even show less artifacts (source dependent).

But, how on earth do I get SVP to open a movie via MPV while downmixing to stereo?
There's a huge difference in audio for the dialogue and sfx/music. Especially jump scares and pop music.

There's no FFDShow like the other setup for AVS+ with MPC-HC and LAV.