But WiiM AMP doesn't cost <100 monies... It is in the region of $€£300-350 - and at that price point I expect certain level of performance...
The load dependency of TPA325x is not something discovered yesterday. WiiM certainly invested a lot of money to design and tightly integrate the AMP...
If there is no PFFB loop, it not very complex (actually you need complex numbers though ;))
I don't have the exact (mathematical) tools needed, but I am pretty certain if you know the function that describes the output impedance of the amplifier and input impedance of the speaker, you can...
Voltage or Power levels are mostly irrelevant for this test. The transfer function will be almost identical if you swipe at 1W, 5W or 20W. It is only close to maximum power that some differences will appear.
It's not irrelevant. Sometimes the other aspects will cancel the uneven response, but sometimes they will enforce it. What one should aim for is, where they have control, have neutrality.
The chase for -80dB THD is meaningless, other elements introduce distortion orders of magnitude larger...
Yes
You will need to set the crossover frequency on the sub in a way that it kicks in when the speakers stop playing. Let's say your mains have -3dB at 42Hz - you set the sub crossover to 42Hz...