My battle with sibilance.

Mijo, Have you noticed that you’ve backed off your original claim that Wilson deliberately engineered a “Gundry Dip” into their speaker’s response (without saying which of their many very different speakers you were talking about).  And that along the way you’ve admitted that the only close to meaningful measure of speaker response has to be done in an anechoic chamber, which you probably did not use in making your own private assessment. No one is saying that a given speaker may not have a dip in its midrange response, due to any number of different factors. So we can let the subject die a natural death.

Most loudness controls on old school preamplifiers and receivers were continuously variable, sort of like bass and treble attenuators combined into one circuit.  So, for example, if you rotated them to left of center, you got a reduction in compensation for the Fletcher-Munson curve.  If you rotated them to the right of center, you got enhanced compensation.  Center detente could be the off position.  (I am not claiming that every Loudness control operated exactly like this; just giving an example.) This allowed for variable correction based on the SPLs at the listening seat.  You see such controls on Japanese equipment right up to this century.  40-50 years ago they were standard on some Harmon Kardon, Marantz, Fisher, Sherwood, etc. gear.  Would you agree that that would be better than one fixed Gundry dip built into a crossover?  Further, the complex network required in the speaker crossover to effect the Gundry dip would require inductors and capacitors, bound to muddy up the SQ.  Let's see, when you personally measured a Wilson speaker, was it in an anechoic chamber?  Could you rule out room effects to help produce the dip you may have observed?  But easiest of all, ask Wilson. I strongly doubt they would introduce such a complex network into their crossovers; it's tough enough to build a linear and transparent crossover network successfully, especially for the 3-, 4- and 5-way speakers built by Wilson.

Mijostyn, you cannot assess whether any speaker has a deliberate gundry dip incorporated into the crossover simply by measuring the response of the speaker, because there are natural dips that occur in association with a crossover from one Driver to another. So you can not know whether that is an incidental artifact of the crossover or a deliberate dip. Further, the very benefit of a loudness control is that by turning the control one way or the other you are adjusting the level of the compensation for the Fletcher Munson curve. Because the degree of loss of sensitivity to bass and treble frequencies relative to midrange varies with sound pressure levels, the adjustability of a loudness control affords much more flexibility  than incorporating a fixed Gundry dip into the crossover network. Did you actually inquire whether  Wilson incorporates a gundry  dip into their crossover networks?

Mijo, There is a long and very informative thread on the "Gundry Dip", also known as the BBC dip, on Hydrogen Audio.  A few of the contributors are speaker designers or acoustic engineers.  For one thing, it is pointed out that a "natural" dip in frequency response in the 1kHz to 4kHz range is not uncommon among 2-way speakers, where the woofer is giving way to the tweeter in that range.  I urge anyone to read the thread for many interesting tidbits, but the consensus is that the deliberate incorporation of a Gundry Dip in frequency response was a passing fancy, no longer espoused or incorporated so much in modern speakers.  As you suggested, the Gundry Dip is or was a way of incorporating a fixed compensation for the Fletcher-Munson curve, the tendency of humans to be most sensitive to frequencies in the midrange and relatively less sensitive to low bass and treble frequencies.  Why we used to have "Loudness" controls.  A Loudness control makes much more sense than a built-in fixed Gundry Dip in the crossover, because it allows the user to adjust compensation according to his or her listening habits, high vs low SPLs.  If Wilson use a Gundry Dip, I don't find evidence for it on the net.

And if your system slices your throat. don't go running to Mijostyn.Seriously, Mijo, good point.  I never knew about the Gundry dip.