Sorry to not come with what you ask for.
But to my ears it sounds like 24 dB/oct
I've made many searches on that question years ago. My conclusion was real speakers have their own cut-off slope. Simulation at 24 dB/oct was not enough and 36 dB/oct was to much. I went with a somewhat satisfacting slope at 30 dB/oct.
Thanks for your thoughts on this.
So you came to a similar conclusion as I have.