Math + Logic + Science = something completely mad...

Jitter for one is bidirectional. It causes harmonic distortion and uncorrelated noise.

  I'm not sure what "bidirectional" means.  Do you mean "sidebands"? 

Jitter does not cause harmonic distortion and that's the reason why is still audible in spite of very low levels.   Jitter only produces additional frequencies NOT harmonically related to root frequency.

Yes, it is causing noise, but it can be correlated, uncorrelated or both.  Uncorrelated jitter is a little less audible.  Correlated jitter is less audible when sidebands are closer to root frequency (frequency causing jitter is low).

It appears that according to numbers nothing in cables should be audible.  Skin effect, that starts at gauge 18 causes extremely small impedance change at 20kHz - frequency that I cannot even hear and where most speakers (being inductive) have high impedance.  Capacitance plays very little role in speaker cables and the same goes for dielectric absorption.  Inductive reactance, assuming both wire gauge and wire spacing of twisted pair to be 50mils, would be in order of 0.1ohm at 20kHz and 0.05ohm at 10kHz (that I can hear).
So, according to numbers, nothing should matter and all speaker cables should sound the same.  The problem is that they don't. 
Perhaps audio, like women, is meant to be loved, not to be understood?

dletch2, I remember long time ago I had inexpensive cable from Best Buy. I believe it was Monster Cable brand. It consisted of two parallel runs of very thick stranded wire in thick clear plastic (most likely PVC) insulation. It was suppressing high frequencies to a such degree that I had to add few dB of the treble on my receiver. Later I replaced this cable with AQ Indigo and magically treble came back (everything else being the same). Indigo was OK, but Acoustic Zen Satori that replaced it is in different league. What surprised me the most was lower midrange (cello, chestiness of male voices) that got fuller. In comparison Indigo sounded thin. I would not even try to express it in electrical terms. Since then everything else changed in my system for the better, but speaker cable is still the same. My hearing was not that good because of age, but I could still hear a difference between AQ King Cobra and Acoustic Zen Absolute interconnect that replaced it (foam Teflon, oversized tubes, Zero Crystal silver). It is cleaner, faster, darker background, more instrument separation. Why? Is it because of insanely good specs (6mohm/ft, 6pF/ft, 20nH/ft)? Calculations would likely suggest that both interconnects should sound exactly the same, but they don’t.

AudioQuest FAQ explains logic behind helical twist on oversized air tube. They stated that in order to avoid skin effect wires have to split into separate insulated strands, but then skin effect still applies since they are in magnetic field of each other. Remedy for that is to place them on the tube. That way each wire is only in magnetic field on neighboring strands. In addition it is interleaved with return wires to reduce inductance and twisted to reduce electrical pickup. That is exactly how my Acoustic Zen Satori is made. Different companies and the same design? Are they all doing it for show? Is it snake oil? I have pretty good understanding of electronics, but getting much deeper into something requires different expertise. I believe that we don’t even comprehend exactly the nature of electrical current. We know that it is motion of electric charge, but how energy can be delivered to load if exactly the same amount of electrical charge comes back by return wire? Why energy flows toward load while AC electric charge flows back and forth? Energy has to flow differently and it does - by magnetic filed outside of the wires (Poynting Field). Do we now understand everything? I remember some horrible amplifiers in 70s that had extremely low THD and IMD (achieved by deep negative feedback) and sounded bright and tiring. Later Transient Intermodulation (TIM) distortions were discovered. That makes me believe that trusting my own ears and cable companies is perhaps better than trying to make sense of it. I selected Acoustic Zen instead of AQ because it is better bang for the buck, IMHO.

Negative feedback increases the bandwidth w.r.t. the definition of bandwidth as the -3db point. However, it does not increase gain at that frequency. We are both going off memory but I am almost certain an amplifier in 1969 didn’t have the IMD numbers you are quoting. I wouldn’t guarantee it didn’t but I highly doubt it. Can you please confirm that in some manner?

No, I remember year and even the store (spending vacation in London) but don’t remember model number or brand. You are right about lack of excessive gain at the high frequencies but I’m not sure how bad amplifiers in 70s were.

Rectification phenomena is not even specific to electrical processes. It applies to thermal processes as well and can cause non linear electrical effects modulating back into the electrical signal, typically low frequency, but you can actually detect it in switching MOSFETs.

Rectification phenomena, as it was described by Analog Devices, at their seminars is an electrical process. Uneven positive/negative slew rates combined with limited bandwidth result in DC proportional to amplitude of high frequency signal (hence demodulation) - at least that’s what I remember. You will easily find few opamps advertised as rectification phenomena free. I’m not sure if slew rate difference is the only reason for rectification, but you can find more here:
https://www.analog.com/media/en/training-seminars/tutorials/MT-096.pdf

The emitted RFI from switching supplies for consumer electronics is fairly low and the efficiency of conduction into a speaker cable would be low. You are correct the feedback gain is high, but still usually bandwidth limited, and while there are RFI noise sources, the efficiency of local wiring and audio wiring for picking up RFI from an antenna standpoint.

It depends on switching supply. Some of them, like resonant mode SMPS, are extremely quiet while crude high current computer supplies pollute everything around. Amplifiers are bandwidth limited (not limiting  already rectified/demodulated signal), and antena (speaker cable) has very low gain for <1/10 of the wavelength. It means that interference will be greatly reduced but not eliminated. I assume we can hear -60dB down from nominal signal level (equivalent to 1mV signal level). As for the standards - you can find them for everything.
Bearingless torquemeters (that I used to design electronics for) digitally communicate from rotor to stator by high frequency pulses and two sets of coils. We had to lower frequency from 20MHz to 9MHz because field intensity was violating standard. Our competition almost lost business (had to stop production) because of that (Navy complained). In the proces we purchased NARDA calibrated EMF measurement device (scope).

dletch2  Let me get to some technical points first.  I brought example of amplifiers in 70s to show that there is always possibility of something we don't know yet.   Designers then believed that NFB is a panacea for everything since it reduces THD and IMD, increases bandwidth and lowers output impedance.  Some transistor amplifiers had extremely low THD and IMD (few zeros after decimal point) but sounded bad.  This deep NFB not only brings TIM but also make amplifier sensitive to external electrical noise, since cable is an antena for RFI.  Amplifier's output has low impedance only for low frequencies and it is an input to NFB at the same time.  Yes, most likely there is a Zobel network there, but no filter is perfect.  We are talking about additional about 40dB of gain in addition to existing about 26dB of amplifier's gain (amp has 66dB of gain before feedback is applied) plus what level down we're able to hear - at least 60dB.  Twisting wires makes cable extremely immune to interference by exposing both wires evenly to external magnetic and electric field (as long as twist pitch is much smaller than wavelength of interfering signal).

As for blind test - No, I did not perform such test.  It happened by chance when after comparing cables I left old one plugged in and next day was able to hear that it didn't sound the way I remember it should.  Also, changes in sound (like fuller lower midrange) were unexpected - reviews never mentioned it.

I agree with you that we have to be objective, but at the same time I believe that we should be humble and accept that what we hear often cannot be explained (or at least easily explained).

I am wrong about 100x higher gain, since only fraction is fed back to the summing junction, but it is in the same level as nominal amplifier’s input. It won’t be as sensitive because of lower impedance, but will still inject noise picked up by long unshielded speaker cable.

dletch2, I'm trying to describe what I hear, while you're saying I have to be mistaken because something like that does not exist.  That is my whole point - you're starting from the wrong side, trying to disprove what can easily be heard.  You also imply, that I wouldn't be able to tell the difference in blind test.  Are you a cable naysayer who tries to fortify it with measurements?  Let me suggest, that placebo effect exist both ways - if you strongly believe that you cannot hear the difference you won't.  Also, there are people with less then perfect hearing apparatus (I'm not suggesting it applies to you) who get very defensive about it making crusade against cables on cable forum.   I'm sure my hearing, even at my younger age, wasn't as good as hearing of many other people and I would never dare to tell them what they can or cannot hear.

Those amplifiers had lots of feedback, but very poor gain bandwidth product. That was why they had bad distortion products, and why they had poor output impedance at high frequencies.

They had excellent THD and IMD.   Designers were sure they have to sound great.  In 1969 I almost bought inexpensive 100W amplifier modules with incredibly low THD and IMD.  NFB increases bandwidth and reduces output impedance.  40dB of negative feedback will reduce output impedance 100 times.

Even when RF gets in, short of causing oscillation it may not even do anything.

You don't even need nonlinear element, like diode, to demodulate.  It is enough if amplifier's slew rate for positive and negative signal is different to demodulate, for instance, AM radio.  It is called "rectification phenomena" and is very common to opamps.  

Yes, high impedance nodes are sensitive, but output of an amp, in spite of lower impedance (many ohms at high frequencies), is an input to the amplifier with 100x higher gain (for 40dB NFB) than amplifier's normal input.  We want to reduce phase delay (improper summing of harmonics) at high frequencies caused by limited bandwidth.  For that we increase bandwidth to more than 100kHz  (my AHB2 has 200kHz@-3dB)  getting very close to AM radio stations, not to mention switching frequencies of SMPS present everywhere (computers, TV) and even LED bulbs.

bruce19 
I replaced banana plugs with spades by crimping them first and then soldering with WBT 4% silver solder.  I applied heat to end of the wire first to prevent spade expansion and loosing oxygen free connection.  That way it's oxygen free and won't move.  Plain crimper worked poorly so for my AQ spades I borrowed from local HiFi store AQ crimper and it worked great.  Both look exactly the same - likely difference in quality.
https://www.audioquest.com/accessories/tools/aq-ratchet-crimper
4% silver solder is probably overkill, but it is doesn't cost much.

Measuring cable's inductance or capacitance with your LCR meter might be inaccurate since you measure distributed inductance or capacitance.  One affects another.   I believe there is a way of measuring it, by doing it at two different frequencies and then using formula to calculate it (inductive reactance is proportional to frequency, capacitive reactance is inversely proportional to frequency) .  Unfortunately your meter measures at one frequency only.