I did the swap, and they were amazed. Both started discussing how they would never have believed changing cables could alter the sound. The change was quite evident, and we discussed at length the effect on the sound holistically. I didn't have to prompt them in any of this. I began to educate them that a person can do this with all the cables in a system to tailor the sound.
There is sincerity & innocence in your post & thanks for being such a good audio host but you cannot be so naive as to think that the mere change of audio cables will change the sonics simply due to the materials & construction of those audio cables. Doug, you've been around the block many times to make such a statement or even a post....
BTW, I do believe that cables make an improvement but I'm more cautious. just to let all know that I'm not an upgrading-your-cable hater & neither do I want to shame people who've spent a lot of money of their cables.
You didn't mention which cables you swapped out but I'm GUESSING it was the pre-to-power.
You have a passive pre & it's output impedance's interaction with the interconnect cable will have to be considered. There might be other cable-electronics interactions in your system esp. if your sonics are changing that much by the mere changing of 1 set of interconnect cables. I find this to be a bad thing in your system - your system is too sensitive to cables.
Interconnect cables & even speaker cables do have changes in the sonics but they are (very) subtle & not as dramatic as your post makes it out to be. There's something wrong in your system where the cable is loading your electronics & creating a low-pass or high-pass effect.