Capacitor

As someone else mentioned the values of every component in the crossover are the more important in the voicing of the speaker, taking into account the character and limitations of the drivers used together, than the quality of the crossover components (well almost always).

Could a better engineered crossover for a particular group of drivers, taking into account their measurements, particularly breakout, frequency response, impedance curves, anti-phase measurements with lower grade components beat out a higher quality crossover in parts? Absolutely.

However, recreating the same circuit (within reasonable tolerances) with better engineered components that will introduce less noise, not choke transients, and reveal more detail is almost certainly going to produce a better sound.

How much to invest, and where the diminishing returns lay, is how well the drivers could possibly play together, how the cabinet works, and the crossover points implementation on the chosen drivers...

Duelund JDM Silver .01uF bypass cap - yeah, you might want to research this, it’s a gem of an upgrade for little outlay.

As Rick knows, that JDM Silver was one that went into my Moab crossover upgrade. Along with Jantzen Alumen Z, Jantzen Premium ELKO, Path Audio resistors and Goertz Alpha Core inductors. Mounted on eMat and BDR Shelf, on Townshend Pods, and treated with TDF, fO.q tape and Herbie’s Grunge Buster, all mechanically crimped direct and soldered component to component with Cardas Quad Eutetic.

Original crossover values were maintained, as determined by measuring all the stock crossover individual component parts. The result is easily my greatest mod type upgrade and exceeded all expectations. Compared to the Talon X crossovers professionally built by Michael Farnsworth and that cost almost as much but provided a barely noticeable improvement, this crossover upgrade transformed the Moabs to a whole new level, night and day.

The question of what is ’best’ is as far as I’m concerned a chimera. A phantom. A ghost. The only way to even begin would be to try all the different parts. Inductors alone on this cost $300. For one set. There were at least three possible contenders. One of them, Deulund, would have cost much more than the $2k total cost of what I have. The capacitor review sheet that was used to select mine had some hundred caps on it. Resistors, at least three, probably six contenders. Then there are the combinations, permutations.

What I did, weigh all the listener comments, and selected what I thought would get me darn close to cost no object for a fraction of the COTA price. I will never know how close I got. But, guess what? Don’t care! 😂🤣
Full crossover details can be found here: https://systems.audiogon.com/systems/8367

barely noticeable improvement, this crossover upgrade transformed the Moabs to a whole new level, night and day.

I was hoping the new Mundorf high priced caps would have transformed my Millennium tweets into something VERY XXP.
But sadly onlya  miniscule nunace = wasteful experiemnt, Now on the woofers, yes, there I got more than a  nunace. 
But sadly all the oney I put into new high tech xovers should have gone into my High sens driver purchase.
Now i am behind that budget some $1200++
Maybe you can tell your mids/highs are now more than a  nuance superior, while others listening o your new xovers may say 
*eh, well...i guess...*
To others thinking upgrading xovers is going to make your speakers into some all new, you are going down the wrong path,. Save your cash, go xover-less high sens.
based on a  painful experiement. 
Even Troels with all Z caps = xovers cost more than the drivers,,!!!!!...
Can not match a  high sens performance in mids/highs. 

$2k total cost of what I have

~~~~~~~~~~~~~~~~~~~

Thats about close to what I spent in the Mundorfs.
For $2k
you can get a pair of Neo High Sens AC1A’s from Voxativ.

Even Troels with all Z caps = xovers cost more than the drivers,,!!!!!...
Can not match a high sens performance in mids/highs.- mozartfan

How much to invest, and where the diminishing returns lay, is how well the drivers could possibly play together, how the cabinet works, and the crossover points implementation on the chosen drivers... - rixthetrick

@mozartfan - There's no surprise superior engineered drivers will produce a better result.

Yes, it is more complicated to know where and when to spend the money to get the best results for the investment. I am assuming you didn't just upgrade the capacitors and nothing else in the crossover networks? 

There's no blanket statement for crossover upgrades in the many varying speaker designs out there, on that we wholly agree.
Though, depending on the parts used, upgraded parts can be as results varying as using different ingredients in a recipe - substantial differences.