> For some reason it didn't work on the A-120 -- had no effect at
> all. But I only installed the resistor, not an extra capacitor.
> I thought the latter wasn't necessary, but maybe it is

Lowering the input resistor should increase the audio level in any case.
Maybe you modified the wrong resistor. It's R15/22k for the old version of
the A-120 (with single transistors) or R19/10k for the new version with
transistor arrays (CA3083). In both cases the resistor is connected to the
center terminal of the input attentuator.

Best wishes
Dieter Doepfer