Smooth treble

Smooth treble is NOT rolled off. Smooth is a perfect term for the phenomena. Smooth means it is 'not jagged' thus it is clear, precise, and more exactly what the original sound was.
I can most easily hear it on a good soprano recording. Some sopranos have a silky sheen on top of the voice. (Sinead O'Connor is one, Emma Kirby another)
Smooth treble will reproduce this exactly. not-smooth treble will make them sound edgy, scratchy, as if they have a slight sore throat.

the bright, brilliant and super transparent treble many people want when they are making the transition from "mid fi" to "high end" audio

Agreed. However, I would replace "super transparent" with "super prominent".

Smooth does, of course, mean there is no jagged & annoying response in the region of 4-9kHz, usually mistaken for "transparency"... as in, "wow, a veil lifted and my jaw fell through the floorboards". I've heard that said of a sibilant set up. Regards

Smooth means not course or irritating during extended listens.

"How can one get this smooth and refined high frequencies without severe treble roll off? "

For me it was a progression of things , of course the recording is the number one component .

I started by ditching metal tweeters in favor of soft dome or organic tweeters . As mentioned above ribbons will fall into this catagory as well .

I then ditched my SS CDP for a tube unit .

Then I ditched my SS amplification for tubed .

And finally finding a tubed int. amp with good top end extension .

It was a process of changes that taught me things along the way . This is not the only way but it is the way that works for me . And other things were accomplished along the way that increased my listening enjoyment .

While I believe that the smooth treble aspect can be accomplished with metal tweeters and SS components , it may come at a greater cost and may or may not give you the desired overall sound that pleases you .
For me it did not .

Good luck .

It is all to do with waterfall...driver resonance is what messes up treble. Soft domes treated with damping material on the fabric work well.

It is often a fight between on one hand stunning brochure quality frequency response plots versus a not so perfect treble rolled off above 12 Khz. If you let the frequency reponse plot dominate your design choice then you inevitably end up with a metal or very stiff type tweeter and that is how you get a waterfall with resonance.

Most people know as a rule of thumb that soft dome tweeters are smoother than metal ones...now you know why.