Since my speakers are not exhibiting the problem, and considering I'm not in a position to have my (to me) precious speakers "at large" for this type of testing, I'm hoping someone else is willing to volunteer their ELACs, especially if they've swapped them out and they're in storage.
If I could spare one of my speakers (or both), I would have sent them to Amirm for testing, to see if they might show better results than the 2020 sample.
Of course, others in this forum have also reported issues with their speakers, and some have reported no issues.
Evidence seems to suggest this issue is batch related, and maybe (hopefully), ELAC has corrected the issue by the time mine were made, and I was just "chasing ghosts" because of my entry level Yamaha's own distortion or issues with Bluetooth clipping my sine waves. In my previous post I indicated I've "cleared" my speakers on a different system with a clean sine wave.
I've also played the "official" Eva Cassidy track through them (even with the Yammy R-N303, a nice but entry level system), and have no issues.
That said, I think it's time to start kicking bass and taking serial numbers:
Hopefully people here will report whether they've seen the problem or not, and post their serial number ranges like I'm doing below.
Maybe you can state when and where you bought them in addition to the first 8 digits of the serial numbers.
My speakers have no issues I've heard yet. I purchased them in May of 2022 from Amazon Warehouse (open / missing box, but like new).
Serial Numbers: 31970E00xxxx (Last 4 digits redacted to preserve their privacy.)
I haven't found any info on how to extract a date code or any other info from the serial numbers, so if anyone knows that, maybe they can pipe in here.
I'd bet there would be some commonality between those with distortion and those without, and maybe we'll find evidence that ELAC fixed whatever problem they had with the mid / tweeter assembly. (I bet it's the mid getting a little chummy with the tweeter and getting overly excited when it reaches it's resonance in that 500 Hz range. I'm sure if it was mechanically rubbing or being interfered with, that would reflect in the impedance chart.)
Sorry for reviving this old thread, but these speakers are still out there and selling even cheaper than when I bought mine two weeks ago! So, we can expect new and future owners to find their way here if they want to learn more about these speakers.
Dan