FWIW, my opinion is that the best response so far is the one provided by JAllen. It provides plausible technical rationale; it provides perspective on matters of degree; and it provides perspective on predictability (or more precisely, on lack thereof). I say that as someone who approaches many controversial and/or technically inexplicable tweaks with a high degree of skepticism.
I would also note that it does NOT follow from his comments that if no difference is heard it necessarily means that the sound quality and musical resolution of the system, or the listener's hearing for that matter, are deficient. As I implied in my earlier post, the ability of a system to resolve musical detail, and its ability to resolve hardware differences, are two different things, that do not necessarily go hand-in-hand, and may even be inversely correlated in some cases. Many technical examples could be cited to illustrate that, involving variables that are dependent on the design of the specific components. In this specific context those would include noise rejection characteristics, RFI sensitivity, intermodulation effects, energy storage capacity, current draw fluctuation, sensitivity to voltage variation, and many others.
Some examples involving other contexts, that perhaps illustrate my point more clearly: Speakers having impedance characteristics that are hard to drive will be more resolving of amplifier differences than speakers having benign impedance characteristics, everything else being equal. Preamplifiers or source components having high output impedance will be more revealing of interconnect cable differences than those having low output impedance, everything else being equal. Resolution of differences between digital cables will depend on the happenstance of the relationships between cable length, signal risetimes and falltimes, cable propagation velocity, component susceptibility to ground loop-related noise, and how closely the impedances of both components and the cable match. In each case, the ability of the system to resolve hardware differences is affected by variables that have no particular relation to sound quality or musical resolution.
Regards,
-- Al
