Just a few more side by sides (all PFFB, albeit the Fosi is 3255 BBTL in addition to PFFB and runs at 48 V, and the PA5II and 3e A5 are 3251 and both run at 36 V, and yes, they are a lot more expensive while the Fosi is not considering its power):
The Topping and 3e are in a class of their own. The Fosi is actually quite good at 1 kHz for HHD (highest harmonic distortion). The Douk sticks out with it high HD3. Its HD2 is excellent (but that is easy to achieve), and its higher orders, i.e. HD5 and up, are actually a tad worse than the Fosi).
When you look at the multitone IMD, you can see that the two champions have about - 120 dB "grass" at 1 kHz, slowly rising to -115 resp. -110 dB at 10 kHz. The Fosi is at maybe - 118 dB at 1 kHz, rising to -100 dB at 10 kHz. The Douk's rise begins at well below 1 kHz, hits - 100 dB at 1 kHz and - 80 dB at 10 kHz.
Two-tone shows the same picture, the champions achieve - 80 dB lowest order products, the Fosi -70 dB and the Douk -60 dB.
The Douk clearly has the lowest PFFB loop gain and the lowest loop bandwidth. That is achieves low THD at 1 kHz (with the exception of HD3) is amazing, and that it almost perfectly load invariant is close to a miracle given that the loop cannot be of much help at beyond 20 kHz. The HD3 probaby originates in the preamp or is down to one component in the power amp acting up.
Finally, the HF THD+N, fully in line with the previous results: