I wonder if you have seen similar, or would want to experiment with this kind of load. And it would be interesting to verify that Class D with PFFB have no problem with this (unfortunately I don't have a hypex or purifi amp to try).
Yes, I have a dummy load with similar impedance plot with negative phase above 5kHz and the A07 amplifier has similar FR issue with it just as you have shown. UcD and NC do not have this issue. As I have mentioned in post #1, the real reason is in different circuit principle of UcD and NC amplifiers compared to hysteresis switching principle (TPA3XXX)
It is a self-oscillating circuit with phase-shift control using the reconstruction filter L1,C1 and phase lead network R3,C2. The frequency of the self oscillations would be at the point where loop gain phase shift is 180°, see plots in Fig.3.
PFFB itself added to TPA3XXX does not help much to fix the issue. It is only a partial cure. A total cure is UcD/NC/Purifi topology with feedback taken only from the load and not in the mix from the load and in front of the LC filter, as is done in TPA3XXX PFFB amplifiers.