I think the more common term you are talking about is “horizontal polar pattern with respect to frequency”. I’m quite surprised to learn that you find the 8361A makes a narrow sweet spot. Are you using them vertically? I think vertical will help. But after all, the main driver is coaxial, which has among the best neutral polar patterns compared to most current multidriver designs.
Ironically, my incredible Dynaudio M5P mastering speakers are also “one person speakers”. Their polar response with respect to frequency looks like a Christmas tree as my friend Thomas Lund likes to say. And I have to agree. Despite the incredible vertical layout of the drivers. Must be something to do with diffraction or not optimum baffle width? It’s something I have to live with. I can put one other listener behind me and looking over my shoulder and they will hear more or less a good stereo image. But anyone sitting beside me gets one side dominant, and yes, the speakers are toed in.
But to repeat, honestly, I think the Genelec 8361A should have a much wider sweet spot than you are describing, if you are using them vertically.