audio circuit: zener clamp
The zeners look like they're clamping the Gate-Source voltage to prevent damage to the output transistors. Looking at the datasheet for the 2SK134, the MAX Vgs = ±14V, and the Zeners are 12V, add in the forward voltage drop across the series diode, and it probably limits the Vgs to just under 13V.

It looks like if you happen to go over that 13V the zeners will save the output transistors and the feedback should limit the signal in the preamp stage.