What matters most in speaker design?

Everythig you said.

Nrenter
Do you want to build some speakers? Just curious how you decided to open this can of worms.
To design, you need a very good understanding of how the individual drivers will react in what type of enclosure, you'll need to understand lobing and how crossover frequency curves that behavior, you'll need to understand cabinet difraction and how crossover curves that behavior, you'll need to understand impedence swings and how crossover curves that behavior, you'll need to know time alignment and how crossover/cabinet design helps alignment, you'll need a good understanding of what different crossover slopes sound like and mixing sensitivity of drivers and which drivers you can pad and which drivers you cannot, driver resonances, controlling phase, on axis response vs off axis etc, etc etc and I can go on. Driver choice... not the cost is very important, but the real deal is understanding how to use them. Crossover design is number one, driver choice number 2, cabinet is number 3. It is possible to find drivers with very smooth rolloff that require minimum crossover that can make GREAT speakers, but hand someone some speakers with difficult curves and thats were you seperate the men from the boys.... unless your a woman, then the girls from the women.

Drew, I'd like to hear the Plutos and Orions.

Behavior at 30 degrees isn't enough.

What does this mean? Of course HF need a super tweeter with a hi-pass filter

You can see Zaph's distortion/ FR / measurements Here

What do you think about Seas W12CY003
or
Seas ER15RLY