Not sure I understand, is it a matter of having one resistor to the ground vs having one on each channel(so 2 in the adapter)?
In any case, I'm curious. I almost never bothered testing more than 2 or 3 values on my IEMs, being already convinced in my head that the results would keep the same direction based on the impedance curve. So it could also be that I missed a very real behavior all this time. For all I know(don't remember), I got measurements like yours sometimes and discarded them thinking I had messed up with some contacts or whatever.
Or maybe there is something about the amp acting up at certain loads and voltages? Or some resistors get hot even with this little signal and the specs change rapidly as the sweep goes through?(I'm totally fishing).
My point is, doubt me at least as much as you do the adapters
. And thank you for looking into it so fast and being so open minded.
I'll go dig into my stuff and test my old ER4SR, but maybe not this weekend because, life.