How much does a Zobel network and its components affect the sound of speakers?

If they impart no sound character, why have some people commented about an improvement in exchanging components of the same values?

Confirmation bias is rampant in HiFi.

He also used cryoed Caddock MP resistors and Hovland mylar capacitors

Replace the Mylars with PolyPropylene and your problem may likely go away. It still astounds that builders compromise a design for a few pennies. Or is it ignorance?

Additionally, there is space, so you may want to try a felt donut around the tweeter to reduce off axis energy and also ameliorate diffraction effects at the box edges.

The amplifier / cable combo [don't be shy tell us what you have] may not like a certain combination of load and drive request.

There may be a resonance due to cone breakdown.

If the tweeters had ferrofluid, it maybe toast.

If there are any electrolytics in the XO, replace them with PP.

Do you have any room treatment? Or is it a far too typical plaster box?

Finally, some recordings are not playable on systems due to Monte Carlo accumulation of faults in recording, playback electronics, cables, speakers and room.

@pmboyd Another possibility could be to replace the parts with cheap value matched equivalents. Should let you evaluate the impact of part quality while only costing $5. If it makes a big difference, than spring for the duelund cap replacements.

@signaturesound Thanks! 

As I stated, I already have the Dueland networks -- the Merlin Master RCs. Evidently some people have replaced the Dueland caps with Jupiter, however, no one has responded to my queries as yet about the difference in sound. In one instance, a chap used a Jupiter in lieu of a Hovland, but he seems incapable of articulating the improvement he notred in specific terms. 

Another possibility could be to replace the parts with cheap value matched equivalents.

Drivers vary more than can be detected with matching caps.

Additionally, ALL of the PP & PS caps I've bought and tested in past few years far exceed their tolerance spec.