So I'll just be measuring the raw fr output of the DAP? I'm not sure what you want me to look for exactly.
Don't all sources measure essentially flat?
I read a review saying that it may just be the R6 Pro II lacking voltage/current swing for more demanding iems/headphones resulting in less bass slam
If you connect the DAP hp out to the measurement line input in parallel with your choice of earphones, you are measuring the loaded output of the DAP when driving your choice of earphones, i.e. the electronic FR as actually received by your earphones.
No, not all sources measure flat in this situation. If they did, you shouldn't hear any tonal differences between sources with your choice of earphones.
It would depend, at regular volumes, on the relationship between the output impedance of the amplifier and the impedance response of the earphones.
If the former is not negligible (i.e. near zero), the circuit is a voltage divider between the output impedance and the earphone impedance. The output FR then takes on some of the shape of the earphone impedance response curve.
Now, most sources do actually have near zero OI, and many dynamic and planar single driver earphones also have near flat FR. Either of these conditions can result in a flat measured FR even when loaded, but it is not a given.
At extreme volumes you might get a change in the FR again, but then you'd also be seeing a massive increase in THD if so.
You may test that last hypothesis by cranking the volume when doing the measurement, assuming your line in doesn't clip.
And if that ends up frying your earphones' drivers, that might alert you to the possibility that these DAPs have plenty enough voltage / current swing to blow earphones up before losing gas