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

Rich, My MMIs are Serial numbers 4168, if that tells you anything. Your information is interesting, but it tells me nothing specific about how the RC networks and their particular components affect the sound, nor anything about how swapping components might do so.

@signaturesound That makes it sound like a worthwhile upgrade to any speaker. Could you also say what value the resistor is in the zobel circuit? I'd be interested in building a pair and trying on my coincidents. 

@pmboyd  - Sorry, hard for me to comment on something I have not tried (trial/error listening of RC Networks with various different part types/designs) as my experience is limited to the Merlin Standard RC network, the Merlin Master RC's. BUT there are other folks making RC network (Zobel) devices. One example I can think of off the top of my head is the Walker Audio High Definition links - these are a Zobel network (which I have heard, but I can't accurately recall the sonic effects they had as it was more than 15 years ago that I encountered them). So, there are other things out there to compare (the Merlin RCs) them to (if you can't DIY your own)... I will say I think that their effect DOES vary from system to system, but generally when you are damping/squashing RF that is at the speaker terminals, you are removing a layer of fuzz/haze from the sound. My thinking (2 cents) is that the resistor type plays some role (as you want something that is non-inductive and acts as a good resistor at RF frequencies) to the sound of the Zobel. The capacitor IMO is the item that has the most impact on the "sound" of the RC network. Apparently a good sounding cap for audio (coupling/bypass) use makes a good cap for a Zobel based on what I hear between the Hovland cap based RC and the Duelund cap based RC (which has a bit more cleaner/natural/organic character in my experience). - Hope that helps some.

@cal3713 The resistor value is 10 ohms (its written right on the part if you look at a pair of Merlin RCs) :-) FWIW, I use old Merlin Std RCs Networks on various speakers from time to time. The magnitude of the effect they have varies (can be very subtle to more obvious), but these days with the amount of RF floating around every home from wireless devices and switching power supplies some amount of that is bound to make its way to one's speaker terminals and it can't hurt to try a Zobel/RC to see if it can help mitigate those effects to some so nice benefit. This does assume the speaker and/or amplifier design you are using does not already have them incorporated into their designs. In my experience, most amplifiers do not have Zobels on their outputs, a few do however which is why Bobby P left the RC networks external to the speakers (doubling up RC/Zobel networks in the amp and at the speakers may make things worse, not necessarily better).

Happy listening,Rich

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.