You're absolutely right. My comparisons, though done with volume adjustments, are as subjective as can be, and they are certainly no proof that the SACDs are indeed better. Maybe I should have added that they sounded better to my (ageing) ears and on my system, and that it's always very much a matter of personal taste.
When I was comparing the various versions, however, I was hoping that my XRCDs and K2 would sound just as good as, if not better than the SACDs, so I could sell the latter to an SACD-loving friend of mine...