What matters most in speaker design?

Resell value and brand recognition to be able to resell locally. ;)

Well the thing is, loudspeaker design is system design. Many an enthusiastic amateur has started out with "the best woofer, the best midrange, the best tweeter, the best crossover, and the best enclosure", and ended up with only an okay speaker. The professional designer takes everything into account at once, including in particular how it all works together.

I would say a good match between the drivers is necessary as a starting point, but in my opinion crossover design is the heart and soul of loudspeaker design. Cabinet design and construction matters, but it's not that big of a challenge in most cases.

Duke
dealer/manufacturer

If you are aiming to design one, go for it. Start from low cost speakers first around $20-40 each then enjoy and be proud of your hard work. Anyhow, a great completed set DIY from PartExpress can cost up to $900/pair while you could preferably buy a nice used set here and you can easily resell later.
Well, as you already know, a great set of DIY speakers can sound awful with a bad cross over design. A decent set of speakers can sound nice with the right x-over design. Guys from PartExpress.com can help you with more details and ideas. Sorry I can't give you a direct answer to your Q because I find that each one of them is crucial and somewhat related to each other.
Good luck.

Duke, a lot of the Klipsch Heritage speakers used to have the 3 dB point at 17.5 kHz. I assume this level helped smooth out the sound and mitigated some of the horn harshness. I noticed your speakers quote the same number under the category of system bandwidth. I think this top end is mostly a tube "limitation" and you mostly recommend tubes for your speakers don't you? Is that what you're getting at - a speaker for tubes? We're always quoted 20 to 20 as the range of human hearing. Are those last 2.5 kHz nothing but irritation? :) I always read with interest your posts about speakers and the obvious expertise. I'd like to hear a pair out here in the southeast.

C - crossovers.

Cabinetry, ala veneers and finish rule the day, with exotic drivers following closely behind. Cetainly a smart strategy, as this hobby is one where people listen with their eyes and their wallets.

However, in my own experimentation, by varying the cap/coil ratio, I could make my own loudspeakers become as forward and white sounding as a pair of Lumenwhites or as lush and full-bodied as Vandersteens, and of course, anything in between those two points.

Along those lines, the unfortunate thing is in this day and age, we are as far away from having competent people designing crossovers as we have ever been. Why? So many of the people designing loudspeakers today are deficient in mathematics; to the point where they run away from it.

For example, one loudspeaker manufacturer who I know, and has won awards at shows got his crossover design from a "friend" in the business. When I looked at it, the numbers were so far off, I jumped up to ask him how he came up with such off the wall(being kind) values, which is when he told me. Knowing the friend's talents lie in many areas of loudspeaker optimization, but not in designing (although he surely can upgrade/mod them to improve sonics) crossovers or mathematics, I approached the friend about the design, and he admitted he knows almost nothing about how to design a crossover. Where did he get these values? He kind of mimicked another speaker on the market he liked, but wasn't 100% sure if he was using exactly the same values. My money says that given the difference between commas and periods on either side of the Atlantic, a decimal point got lost in translation. What's worse is that even after a fundamental and major redesign of the speaker, the crossover has not changed one iota, "I have a crossover that works, and am afraid to mess with it." Really? I've told him the more appropriate values several times, but hey, the parts have already been bought and paid for.