Excessive sibilance and edge....treat room?

Silk dome tweeters are 'slow'. They stretch out the HF resulting in your issue. Soft and sibilant, and therefore with an edge. Aluminum, Beryllium, Titanium, Mylar. For in-between, Phenolic is a good compromise. Soft tweeters suck!

I think you misunderstood me there. I gave a list of the opposite of 'soft'. If however you didn't misunderstand, then no, I'm not kidding. I hear the same thing with soft dome tweeters.

Okay, if anyone has something to learn in this hobby, it's definitely me! I would not consider soft dome tweeters to be harsh. Looks like there's a lack of communication here which could very well be attribute4d to me. Sibilance as I may erroneously understand it is a lack of transient precision or clarity. Hard domes or ribbons/planars are unforgiving toward the signal further up the chain, hence a tendency to slur HF transients resulting in sibilance. Soft domes tend to cover the sins of the aforementioned but incidentally fall by nature to the same/similar fate?

I have a vague sense we're on the same page. Defining sibilance as too much precision or clarity seems a bit oxymoronic, though. How can you have too much? By saying HF are being stretched to sibilance with soft tweeters to me is more accurate since distortion is after all the result which is in the end a loss of original content/experience. You could say distortion is an addition to original content but still, I would not use terms that contradict that. I'm pretty sure I'm in agreement with the rest of your assessment.

Another way I could state it is to say that HF transient response is audibly better with hard vs soft tweeters in that HF are more highly defined with them, thus less sibilant. The microscope analogy is a predetermination by the recording venue and therefore the effective original. Whatever it is is the playback goal. I don't see how that relates to this topic. I think you are misplacing subjectivity with communication. Language is after all the largest barrier. Having only a keyboard to communicate with exacerbates this problem to the extreme imo. Probably the main catalyst to the subjectivist disposition so ubiquitously held in virtually all the forum sites. Consensus took a serious back seat with the inception of the internet. I can relate to the 'metal' dilemma which is why I like mylar ribbons the best.

You used the microscope analogy as a bigger than life product as it would relate to a recording. Being closer to the subject doesn't necessarily conclude with sibilance. In any case, ime it's been a lack of performance on the amp's part that has been bothersome to me as far as sibilance is concerned. If an amp isn't fast enough to negotiate the important consonants to life like realism, it's a poor design no matter what else it's good at imo.

I should say mylar on aluminum foil such as those on my Kappa 9s. Or the solid mylar domes on the Kef 105s.

There's lots of issues other than amp specifics especially when vinyl is the source but once everything is in order, I really think it's the amp you then are actually listening to. It always seems to be recognizable through any other changes. At a certain level I think it becomes the weakest link. On a descending scale of importance, I place it at the top for the reasons I've stated.

I've seen the Kappa Emit type tweeters on other brands of the same era. Can't recall any names.