I do both, repairing and modding speakers for others mainly, and build new myself. I like to design my own builds myself, even if i know i don't have all the skills tools or budget that pro's have. But designing is half the fun, and i don't want to skip that. I'm in downtime now for medical reasons since a while altough (but things are getting better).
I like single driver fullrange speakers; even if i know the limits of those. But i don't limit myself to that. I designed and made all kind of speakers, for myself or others. I'm an amateur in this, but my designs are rated by friends who have them or hear them so it's not that bad i guess... It's a hobby that i like a lot, not my profession or so, and all i know is self study. I'm not formally trained in anything to do with speakers or even physics. But i know a lot of (good) speakers, some use very often are superclean (Genelec, Kii) so i got good references. Not that I claim mine come close, but they are not that bad, and a lot better than a lot that is commercially sold.
And the knowledge i got trough that helps also to judge a speaker fast without measurments or even listening to that. If you know the physics behind it, you can easely see where claims are made that are not possible, or how a speaker will react to a room. You don't know the details or have all the info, but seeing a speaker tells me already a lot on how it's working and not working. And knowing the basics of electronics helps also, also with other audio gear (amps, dacs, ...).
Joseph Crowe is good in what he does. It's very niche, but it's based on measurements and physics, not audiowhoo. His plans are used a lot because they do what he promises. This kind of bussinesses are all over Europe present also. They choose a niche in music (often connected to a music style or lifestyle) and make speakers tuned to that, mostly handmade small series. And they mostly don't cost an arm and a leg altough custom hand made in Europe is not cheap.