For an unrelated reason I bought a used set(4) of AQ Sorbogel Big Feet and put them under the Calypso just to hear what would happen. The "fuzz" at the peak of the notes disappeared, the notes were clean/clear!!
I keep saying this on many threads. Tubes are microphonic! They really ought to be kept in a separate room/area and away from the noise in the listening room. This is what Pink Floyd does and surely we can respect them for knowing what should be done soundwise to get the most from the great sound of tubes.
It is entirely possible (in your previous observations) that the different XLR cables changed the way the components resonated with the sound in the air in your room or transmitted from your speakers to the floor and up your component stand.
A much heavier flexible XLR cable (compared to RCA) might logically have a dampening effect in much the same way as if you place your hand on something. However, now that you have resolved most vibration issues then the difference from RCA to XLR has become far more subtle - as it should when everytyhing is working properly.