Also my two cents on the importance of the spinorama. I don't th
I'm curious if it's possible to create a number of very similar bookshelves that are largely the same in as many areas possible but with specific dips in DI, for ease of listening comparison, with slightly different baffle shapes, for example.
So if I am reading this debate here correctly, it seems like you want to isolate the effect of DI on sound quality perception, specifically trying detach it from other factors such as distortion. Obviously you can equalize a speaker to have flat FR on axis, which eliminates that factor. To eliminate the influence of distortion, you have two options. One, use the same drivers, or two, use drivers which have negligible distortion.
Being an idealist, let's try to pursue option #1. Unfortunately, the biggest contributors DI are piston size, so the range of different DI will be small. Yes, you could make a wide baffle and narrow baffle speaker, you could make a poorly integrated or well integrated crossover, but you start running into problems when you try to manipulate the drivers into having different dispersion characteristics.
All things being equal, a speaker radiating in a wider pattern will be putting more sound power into the room. So, if you have a narrow dispersing driver and a wide one, both the same amplitude on axis, the narrow one will be having an easier time doing it. In other words, distortion goes down with narrow dispersion and goes up with wide dispersion. At the extremes we can illustrate this principle easily - dipole speakers need to move like crazy to get mid-bass, and tweeters in waveguides can be tiny little domes and still play loud without difficulty (see the 19mm unit in the buchardt.)
So, using the same drivers, you can change it a bit. Smooth vs lumpy, maybe massage the upper midrange a few db, but if you want something very different you need to use different drivers.
If you want to isolate the importance of distortion I might suggest a simpler test - go outside, somewhere quiet and free from lateral reflections, and play the same speaker at different distances, but keeping volume at listening position the same. Pretty easy to do a good test.