In this review about the C-2120 ("Stereo" magazine from Germany, translated by Accuphase)
http://accuphase.com/review_pdf/STER...0_01_2015e.pdf
they say:
(...) It also defines the sound character of the C-2120 and validates simultaneously the HiFi wisdom, which warns from little accentuation. Then the Pre-amp only offers in the “12 dB” position maximum, roomy, light and fanned out sound patterns. Already at “18 dB” it sounds a little tighter and more compact, what can be exciting with pop music, but doesn’t transfer as exhilarating and airy as before, furthermore the mid tones gain a little more severe diction.
“24” dB might allow to enhance consumptively pale sources such as Portis, but the C-2120 in turn looses much of its audiophile qualities
And about the Gain selector on P-4200, also reviewed, they say:
It’s gain selector with the positions “MAX”, “-3 dB”, “-6 dB” and “-12 dB” lowers just the input signal and not the tonal brisance of the “GAIN” – switch of the Power amp.
I keep the gain setting on the C-2420 at 12 dB, and if needed also lower the gain on A-46. I think the gain on the A-46 works by applying more or less negative feedback, so in theory something should change in the presentation, but honestly I never noticed striking differences between the settings.
I try to put myself in a preamp volume range range where the noise is theoretically minumum (not that I think you can actually ear any differences regarding noise in any position). Normally now I am between -30 and -40 on the volume, depending on the source.
Here is a noise graph from an Accuphase C-2120 paper:
