I've listened to a few 2" compression drivers, and have owned TAD TD-4002s for over 12 years--on my K-402 horns. They are the standard by which I judge other drivers.
More recently (last March), I evaluated a Celestion Axi2050 on a K-402 horn in a stereo arrangement with the other K-402 mounting a TAD TD-4002. Since the transfer function of the two drivers/horns in-room were essentially the same, I was able to listen to them in as a stereo pair. My comments are found here:
https://community.klipsch.com/index...on-driver-on-k-402-horn-and-jubilee-bass-bin/
I also run a BMS 4592ND (bi-amped) in my center K-402-MEH--a full-range loudspeaker based in a three-way (including the 4592's two diaphragms) multiple entry horn using dual 15" woofers-- discussed more here:
https://community.klipsch.com/index.php?/topic/161404-a-k-402-based-full-range-multiple-entry-horn/.
All in all, I don't believe that your focus on harmonic distortion (HD) is a good way to evaluate these drivers--at all. In my experience, the factors that are audible that differentiate the drivers are:
1) smooth non-breakup SPL and phase performance above 6-7 kHz (very audible)
2) timbre of the driver vis-à-vis TAD TD-4002s (almost as audible, but requiring significantly more time to evaluate using a variety of recordings)
3) "sparkle" or lightness to the presentation that correlates with the diaphragm's ability to play without breakup with ease above 10 kHz (least audible and requiring many hours of listening to form an impression--greater than 20 hours.
Under this scale, my impressions are the following (fully dialed-in drivers/K-402 horns using First Watt F3):
1) TAD TD-4002: no coloration, no audible diaphragm breakup even at high SPL, and plenty of naturalness and sparkle.
2) Celestion Axi2050: no coloration, no audible breakup on cymbals, etc., but slightly lower sparkle and naturalness, almost not audible even after many listening hours.
3) BMS 4592ND (bi-amped, time aligned): no coloration, no audible breakup on cymbals, etc., but slightly more irregular SPL response around the internal two diaphragm crossover point (~6 kHz). No real audible differences between the TD-4002s and the 4592NDs.
All three have almost no differences in listening quality, especially after using a "phase plug extension" (or otherwise described as a horn throat diverging acoustic lens) that spreads out the polars of the 2" diameter throat, which doesn't affect on-axis SPL response, but significant increases the large off-axis timbre consistency. I would seriously doubt anyone could tell the difference under blind test conditions. I haven't heard any other drivers (including AMTs, etc.) that could match the presentation of these three drivers on a K-402 horn, fully dialed-in.
I can answer more questions about the differences, above and the other drivers/horns that I've evaluated, but this is a good place stop for now.
Chris