That's an easy one: Danny Richie at GR Research. Watch his videos on YouTube, wherein he measures loudspeakers (and listens to them, of course), then designs x/o filters to correct response weaknesses (if the speaker exhibits any such). He also replaces junk (ferrous) parts with audiophile-grade ones, changing iron-core inductors to air-core, electrolytic capacitors to film, and sand-cast resisters to Mills wire-wound. The best in the business.
By the way, he will at no cost evaluate any loudspeaker sent to him, creating compensation filters if necessary, and sell the owner the parts necessary to build the improved x/o. No, he won't build it for you. This is strictly DIY.