I think the answer may lie to a great extent in (a) where in the system the various cables go, and (b) how similar to or different from each other in tonal balance they are. When I gradually shifted from Straightwire Crescendo, a somewhat lean, "bright", more forward cable, to Cardas Golden Reference, a warmer, richer, rounder cable, I found the sound change varied most by which cable came last, and that as a result as between interconnects the one between preamp and power amp seemed to have the largest effect. Certainly the speaker cables had the "last word," and made the biggest difference of all. Recently I have started shifting to NBS Monitor 0 cables, and found the same thing appears to be true. It is harder to generalize on this last change, however, because while the NBS is fuller and more extended and slightly more detailed in the lower midrange and bass, the overall tonal balance of the Cardas and NBS cables is quite close. I agree that changing all the cables at once is preferable, if you can afford it, and if you've chosen a good match to your components.