...the 'passive' devices in series with the signal and probably the parallel devices too. Most crossovers are easy to diagram, with the easiest high-pass being merely a series cap and maybe a parallel coil, while the simple low-pass filter is a series coil and maybe a parallel cap. (The parallel devices make the slopes 2nd-order = 12dB per octave. A third-order filter adds another series device after the parallel device.) Most manufacturers now use good-quality caps (instead of the 10-cent electrolytics of 20-and-more years ago), but you can still do better. I started my capacitor-replacement insanity over 20 years ago with WonderCaps and have used lots of them and Reliable's film caps and MultiCaps, but I think I'll try the auricaps in my Aerial CC3B, which uses 3rd- or 4th-order filters and Axon polypropylene-film caps. Next will be ANOTHER new crossover for my 17-year-old Kindel PLS-As.
I did say 'insanity', didn't I?