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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.
But I’d like to find an expert to upgrade the crossover, speaker terminations and l-pads.
Find an expert? Why?? The crossover upgrade will consist out of swapping out a handful of caps and resistors, soldering them together with some fresh wire if you want. The values of all these parts are written right there on each part. You write down the values, place your order, solder them in. If you want to get really into it you search around for advice on which particular caps to get the sound you want.
Same with the wire. I would use ordinary copper wire by the way. Why? For the same reason I would just swap out caps. You're doing this because you like these speakers. Going to silver wire or different crossover will change the sound you love. Cap upgrades will change the sound as well, but in terms of improved refinement not its tonal character balance like the other stuff will do.
There's some other stuff like fo.Q tape that falls in this category. These things will make your speakers sound just like new. That's probably the best way to describe it. Just the same, only better. Altogether they will probably cost you less than just the shipping alone to have some "expert" do the exact same thing. Only not as good. Your "expert" won't even know fo.Q tape exists. Let alone TC. But hey, don't take my word for it. Go and ask. You will see.