I need a qualified engineer to confirm what's going on here please as I'm shocked that just ONE series resistor in each channel can make such a sonic difference (I think I really am going deaf now, as resistor tolerance would count more to me than type or maker in this case)...
Not sure what qualification you need, but I certainly have experience of this. Not entirly sure if you are asking a question with the above sentence or just saying what you expect to happen. Yes, you can clearly hear the differences in the shunted resistor in this sort of setup. All this was tried ten years ago on the WAD forum with just the same results. Its worth pointing out that you can (and I often do) use a shunt with a conventional pot to remove much of the sound of the pot. I do have to point out that I sugested that this was the best solution some pages ago and was told it "just didnt feel right" or some such. But hey ho.

A 'shunt' passive attenuator has ONE fixed value resistor in the signal path and each volume 'step' has a different resistor each step to 'load' or 'shunt' the signal further to adjust volume. This means the input and output impedance varies with each volume 'step,' doesn't it?
Yes.

It just strikes me that a 'shunt' type is only half done, but if the varying load with each volume 'step' to both source and amp doesn't matter, then it's not a problem at all I expect.
Again, yes, as I said some time ago, if you need fixed resistances in the input and output, then the shunt is not the best choice. Likewise if you need max output to be 0dB then a shunt is no use, if neither of these, then it can and does work well.

You may have thought that places like HiFi collective sell their wide range of components just to pander to fools, no, these things do matter, the reason manufactures don’t normally use them is simple, cost and the effect on their profit margin. If you looked inside Gary's 300b amp, you would find that the resistors in the signal path are all better types than the nasty metal film that sell for 1p a resistor, likewise the caps and so on. I use metal film in places like the power supply where the cost of something better is more than the benefit they bring. I dont do that just so I can charge more for the amp, I do it because it sounds better.