You know, for all the discussion on that thread, nobody posted the obvious test:
Hook the different AC main cords into an A/D converter. No, I don't mean the power jack, I mean an input. Of course you would need an appropriately rated pad in the chain so as not to destroy the world--I think a 100K/1K/100K 1W U-pad would do the trick.
Then just record the signal on the AC main. If the cable is acting as a filter, it will be readily apparent.
In addition to that, the regular tests for resistance, capacitance, etc.
No subjective listening, recording of playback through monitors, etc., required.
Edit: if the current draw needs to be higher to demonstrate the effect, chain a pair of pads, the first with less resistance, say 1K/1K/1K 10W, the second as above.