There is no data because it would not look nice on the paper. Based on
@amirm measurement, voltage drop from 4V to 3.6V when AP output impedance changed from 40 ohm to 200 ohm - results in Topping input impedance of 1.44 kohm - simple circuit maths.
This is very small input impedance. I would recommend nothing below 10 kohm. It affects distortion, and the low frequency corner, if the sound source has output coupling capacitor, which is very usual.
In my designs, I use 47 kohm or 100 kohm input impedance, very rarely 10 kohm.