Still my favourite single-BA IEM is the Prelude. It's the best I've heard for its kind. This one mad engineer from Turkey is obsessed with improving it even until now (a decade later). He's another devoted craftsman with a cool vision. Warbler has only one IEM in their lineup currently, Prelude and that's it
Everything is taken into account for each ear. The angle, distance length, even the surface material. All to create the best most natural sound possible for human hearing
That'd be something I'm interested as well. But then the concern I think would be implementing the crossover, and blending them all together. With a single BA/DD the cohesion is very high, but comes with some deduction in reach and finesse. Inversely for multi drivers, I think their challenge is to solve the crossover problem, and create a more cohesive sound
That said, I think Vision Ears newer EVE21 might be using DD. They've been toying with the idea through the development of Ely and Meister. Excited to hear how it's going to turn out