Let's also look at distortion for the newly added WiiM amps and the Stereo Fosi, which happens to run at 32 V same as the Douk and the A07 I compared earlier.
HD at 1 kHz, the regular Wiim without PFFB has the same - 92 dB HD3 the Douk has with PFFB, but the Douk is about 5 dB lower on the higher harmonics. The Pro Wiim cleans both HD3 and the higher HDs up with PFFB and is clearly a better performer than the Douk. The big surprise is the Fosi which is a lot better than the A07 we looked at earlier. It surpasses the Douk on HD3 (-99 dB) but is about 7 dB worse on higher HDs.
Going on to IMD, the Wiims and the Fosi all are about 15 dB lower in grass at 10 kHz than the Douk. The Pro dips a little lower in the mids but is a little higher at the top end (PFFB with limited bandwitdh noise shapes the harmonics into that range).
On two tone, for the lowest order, the Douk is at -60, the Wiim at -78, the Wiim Pro at 68 and the Fosi at -76. So again, the Douk is the worst performer, and we again see the HF penalty of not enough PFFB in the Wiim Pro.
I am going to spare you the power graphs. The Fosi on 32 V goes 3 W higher than the Douk at 4 R, so it is more similar to the Douk than the A07 in this respect.
For THD+N at 15 kHz, all four are pretty close. Again, the non-PFFB are ahead, the Wiim Pro next and the Douk behind, but we only have 7 dB spread at 15 kHz / 30 W.
I would really pick the Fosi Stereo among this pack. It seems to reach good distortion performance across the band and the lowest degradation to HF, likely through the use of good passive components and PCB design. It might also be a good testbed for diy experiments with PFFB or composite designs.
At $71 with 32 V and $94 with 48 V PSU (shipped from Germany and including 19% VAT), it is also cheaper than the Douk, alas without the HP filter.