In my board I have additional diodes D8 and D9 connected between the gates of Q16 qnd Q17 and the ground.
It seems that the voltage delivered by the bias circuit is limited by the those diodes to 270 mV.
However, when I set the DRIVE to 100, the voltage on the gates looks like in the picture below (probe x10 already considered):
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So it does not seem to be limited by the diode (GND level is on the central line of the screen).
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The picture of the board is below. The D8 and D9 diodes are above Q16 and Q17, next to L80 and L81 (which are in fact just jumpers):
