Lowering gain by adjusting cathode resistor value or removing cath. bypass cap

[loudthud]

I thought the zeners were there to protect the JFET switch to ground.

[Helmholtz]

I thought the zeners were there to protect the JFET switch to ground.

Yes, see post #9.
The voltage jump at turn-on might be critical.

[Chuck H]

I thought the zeners were there to protect the JFET switch to ground.

That was my bad. I misinterpreted the diode circuit as a potential clipping circuit because it was across the signal and 0V. If you read the course of posts you'll see I found it wasn't.