- What ADC did you use? If it was the E1DA Cosmos ADC, I assume the APU was not really needed for an HPA (no issue with ADC impedance) at this level of noise and THD+N, correct?

The APU notch may be used, as it s here, to provide a more realistic load to the DAC/HPA + the notch allows a much deeper analysis of THD.

- The Archimago notch compensation curve gives the nice and "usual" FFT graph that you show, but it is not really needed either to get the correct N / THD+N results, correct?

The Archimago compensation is an approximation of the (reverse) notch filter, so, as it compensates (although approximatively) for the notch filter, it is needed to get readable THD+N result.

- Any reason why you choose to use the Dolph-Chebyshev windowing?

Audio Precision uses a proprietary "AP Equiripple" FFT windowing function, which they describe has "derived from Dolph Chebyshev".

So: why not to use the closest function from the AP's ?

It shouldn't make a difference, though.

By experience, what does make a difference is that REW should be set to use "AES17 notch" setting for THD+N (and especially for the "N") measurement with the APU

**EDIT:** FFT size 2M is complete overkill.

64k is far enough at 44.1 or 48kHz sampling frequency.

Also, the result is better if the ADC input level is close to 0dBFS. Here, with the compensation, it's difficult to say what the ADC level is, but -9.63dBFS, if that's the ADC level, is far from optimal.