I think of it like a tree with trunk, branches, and leaves. All the energy passes thru the trunk, the power cord, as Robert points out. Next most energy is thru the amp/speaker interface, and next most the preamp/amp connection, and last (the "leaves") between source/pre-amp.
The speaker cables are sort an equivalent "branch" in the hierarchy, to the amp/preamp connection (granted they carry more energy.) So one could theoretically make a case for upgrading the speaker cables right after the PCs. However, I'd argue that even decent gauge but inexpensive zipcord would allow accurate comparisons of cabling on the "input" side of the amp, and because they're not part of that "chain" of components/interconnects, that leaving them for last, after improving everything else, would allow for better hearing comparisons of different speaker cables. Just a thought ;--)
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