Modifying Crossovers

3 things...

1. There once was a speaker designer who claimed changing the quality of parts in a crossover (while maintaining the same circuit design) changed the sound of the speaker. Many disbelieved. So, he installed an upgraded crossover in a set of 2-way speakers w/ an A/B switch, and graciously sent them to nonbelievers to experience first-hand, in their own listening space, with their own equipment. If I recall the results of the experiment correctly, the ability to distinguish "Crossover A" from "Crossover B" was unanimous, and the preference was for the crossover with "better" parts. If the additional expense is "worth it" is always a source of debate.
2. I believe if crossovers were externally mounted (and on display like audio jewelry) the quality of the parts would increase dramatically. And, yes, cost would increase dramatically, too.
3. There is no "best crossover" (including the "no crossover" option). There are only trade-offs.

Several years ago, when I had an obsession with vintage Pioneer equipment, I stumbled onto a set of Pioneer HPM-200 speakers (in pretty good condition) and wanted to see how much performance I would squeeze out of them. I commissioned a gentleman to reverse-engineer the crossover circuit and rebuild it with premium parts. The sonic results were amazing (and the measured performance was impressive, particularly given this is a complicated speaker designed in the 70’s). My whole restoration is documented over at AudioKarma.org. Combined with a minty (and restored) Pioneer SX-1250, a Realistic LAB-500 turntable, and a TEAC R2R, those speakers round out a very cool vintage system. Soon, I’ll want to part with it all, and I’ll probably never get out of it what I put into it,  but the parts upgrades did make a difference. 

For the manufacturer to move from caps that cost less than a dollar each, to ones that cost $5 or more, ends up raising the price at the consumer level by much more.

...and just to be clear, people are generally not upgrading $1 capacitors to $5 capacitors. Just go out to Parts Express or Parts ConneXion, choose a capacitance (like 0.1uF) and look at the range of pricing. Want to spend $431.68 on a single cap? You can do it! Image what a crossover full of premium parts would do to the price of a speaker.