[Silicon]

To sum up the most important facts:-

Power supply gives out 24V but this drops to 17V when the amplifiers are connected.
The manual states that the supply is 24V.

This suggests either:-

a There is a high resistance in the power supply (i.e. faulty rectifier)

b The amplifiers are taking too much current from a healthy power supply.

c Both of the above.

500mA seems much too high for the quiescent current which should be 65mA.

I would be tempted to bypass the bias potentiometers with a small value fuse to convert the amplifiers to class B operation where there is no quiescent current.

It should then work but with some distortion.