The ADC has the best response with a 60k resistor across the secondary to even out the ringing. But it still drops like a fly after 10k.
In general all of your vintage sweeps are rolling off much earlier and steeper than I ever measure here. Not sure why, but why I made the suggestion. You’re getting results up top that look more like early 1930’s inputs.
The ADC in particular may be designed for a resistive secondary load, many winders used to offer loaded and unloaded varieties.
I just tested an RCA bridging input that has a 15dB resonant peak if unloaded. It wants a matching condition 50K; even 70K is +1.
Anything you test with a split secondary winding that shows a dip at one spot in the treble is interesting to look at in PP, pretty much always response is better than SE. All UTC A / HA / LS, lots of ADC, etc etc.
There’s an ADC with split secondary used SE in one Gates preamp, uses one sec and leaves the other floating. I tried connecting it both series and parallel and both look much worse. Looks best of all in PP, best used that way.
Have also seen an LS-10 with one bad secondary half. Response using just the good half was much worse. Response IMPROVED with a resistive load across the floating BAD secondary, which I take as more evidence there’s always stray capacitance leakage to consider.