To take Serge's point in relation to my earlier post: I'm pretty sure the measurements would be within the normal expected range and differences would not be considered critical to performance.
I didn't make a conscious decision to like or dislike either of them, and had equally high expectations of both of them. The rest of the system remained the same. The fact that I subconsciously listened to a lot less music with one amp suggests, quite strongly, that I got much less meaningful musical experience out of my system with that amp in place. Again, my personal circumstances hadn't changed. No more, nor any less access to my system, or time in which to listen.
So, the obvious variables having not changed, I think it's a reasonable conclusion that the amps were the difference. I merely point out that I doubt the Harbeth Challenge(TM) would have flagged up these differences which, for me, have turned out to be fundamentally important.