What does this rail-splitter-like circuit do?
It might be intended to generate a constant load on the 50 V rails. Some (early) switched supplies don't regulate properly at low output current. A circuit like this could guarantee enough load to allow proper regulation. It might have been easier to add this circuit than modify the switched supplies to work properly at low output current.
What voltages do you get on the power rails with no audio and running at a decent volume? If you can disconnect the amplifier from the 50 V supplies what output voltages do they then have? Is there a voltage rating printed on the zener diode?