You need to place the mod between the negative output of the charge pump and the PCB, not between the the power supply and the charge pump.
Really, you want the mod between ground and the circuit, because the charge pump is tricking the transistors into thinking the -9v is ground and ground is +9v, but you don't want it to affect your chassis ground, i.e., you want the sleeves of your jacks to still be able to connect directly to ground without any resistance in the way.
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