Just to stick my two cents in. The RCA pieces seemed to make the biggest difference in ordinary commercial CD players, sub $500 products. Rendering the typical high frequency hash and garble, however slight, back into the musical information it actually is, before the op amps have their way with it. Then I got hold of this solid state Nikko preamp and now the biggest difference made is with the Nikko.
Our Asian markets seem to be much more interested in the RCA than the lugged speaker / amplifier Ground Control and the change in the Nikko's behavior may be a clue as to why this is true. In the other cases I have tried the change was subtle, being primarily in making three dimensional interpretation of the stereo illusion much easier and removing even more of the noise masking, small, wide band wide frequency response information difficult to interpret. This seems to be more of an ease in spaciousness than anything else.
Bud