There's more to crossover design than just the crossover freq.
A drivers' impedance curve affects the crossover filter's output function, so the parts need to be custom selected to match the driver to create the desired acoustic roll off.
Two different midrange drivers will need different crossover parts to achieve the same filter function on the acoustic output, because of the different load impedance curves and different frequency response curves of different drivers.
Making two drivers blend together is more than just picking xo frequency, they must also sing together in time to avoid phase cancellation and crossover has a big part in that phase coherence because the slope of the filter changes the rate of rotation of the phase per freq.
The box size and shape also affects the diffraction frequencies and bass extension and thus the crossover design.
None of these aspects are considered in the premade crossovers, unless they specify the speaker the crossover is designed for. It's really a sucker play, offer for sale the hard part of speaker design that most hobbyists don't know how to do, so they feel confident to buy the expensive drivers. But it will sound bad.
Sometimes sounding bad is OK as long as it's cheap, like garage band PA monitors or GenZ beach boom box/drinks cooler. Parts express is not just for audiophiles.
Not to sound snobby because I'm very frugal, but I do have a good ear and I must normally spend about the same money on a single good tweeter cap as those whole premade crossovers cost. They are made with very cheap parts that sound bad by audiophile standards. But the filters function correctly on a frequency response curve even with cheap parts, and the price is right. But not intended for audiophiles.
It's fine to buy a premade crossover that's intended to work with specific speaker design - say as part of a kit, or as an upgrade to replace the stock crossover in a certain commercial speaker. But usually the crossover is custom designed towards the end of the design process to make the rest of the speaker parts work together to sound good. A designer may even start out with certain crossover ideals as design criteria, like 1st order slopes, or minimal parts count, or no resistor on the tweeter, and then design the speaker around those ideals, but the parts values will be chosen by ear or by computer simulation after the rest of the speaker is assembled and measured. Pretty much impossible to do it backward, choosing drivers and box to match a premade xo.
