Thank you so much for that very articulate description. It seems you value the same things I do and I fully agree that piano is a great test. In the past I've owned the Ref 3 and Ref 5 and felt it was 'fluffy' sounding on piano - that it lacked the combination of weight & sparkle that a piano emits. I've also heard that the Ref 10 and 6 are better in this respect but until I hear it for myself I wouldn't count on it.
I always wondered if the dartzeel would sound synthetic or non-organic in the way that many solid state designs do, but I'm guessing that's not the case.