I consider the Anne-Sophie Mutter recording "Carmen Fantasie" as the ultimate test recording for treble sweetness.
That is an excellent recording - although I am not a fan of DG.
I'm wondering if this silky presentation can be considered as a hallmark of quality for amplifiers or rather a sort of coloration that makes the upper treble soft and "pretty"
Your description makes me think of Luxman SS amps of the 80's that did this kind of thing - I have always steered clear of amps that add coloration...I had one for a while (forget which model but it was high end SS) and it became irritating as you could hear the same coloration on everything - nice for light polite piano bar trio jazz with brushes but it spoiled other genres for me...
I find Mobile Fidelity Sound labs recordings often had this kind of "polish" in the highs on their CD recordings - I found it irritating too - I am not sure how they achieved the coloration...but I stopped buying after I noticed the same issue on albums from different artists.
IMHO, rather than focus on one recording - try a variety of recordings to see if the amp is making an honest uncolored presentation....this may mean that some recordings are too edgy or strident but others will bloom...all depending on what they did in the microphones/mix/mastering process.