That is a good question. In our application, the impedance is still high, it just becomes more capacitive then inductive. If you look at the impedance plot, the resonance is about 20 MHz, but the reactance at 30 MHz is still 2000 ohms, so the gain goes up. On the 15 meter band, it looks like 30K ohms, so the amp runs open loop.
The bottom line is that the design is not that robust to begin with, and what I have made is a patch. I would never have designed the signal path myself this way. I would have use an opamp to drive the transformer to replace everything from C80 to T10.
The fix works, and has been validated by others. It would be smaller, probably not cheaper, but robust.

Howard