I did a mistake in my last post for the value of the resistors of the Vbe multiplier. I used the textbook formula for the Vbe multiplier Vce = Vbe (Ra + Rb) / Ra where Ra is the total resistance between the base and emitter of the bias transistor, and Rb is the resistance between the base and collector. This formula is valid only if the bias transistor has a very large hFE. For low hFE, resistor Rb must be low or you will need a negative resistance for Ra... For hFE=100, the correct value for Rb is 2.2k and the correct value for Ra is 4.1k, in this amp. What is going on in your amp is that the range of adjustment of the bias voltage source is very limited and your driver transistors have a Vbe which is too low for your bias voltage source. Can you measure the actual voltage between the collector and emitter of H760 for different positions of the bias trim pot, for example at 0%, 10%, 50%, 90% and 100% rotation ?