What is the ideal High Freq extension?

My opinion is that the behaviour up to 12 Khz is much more important than between 12 and 17 Khz and frankly above 17Khz it is mostly for the "bats & dogs".

All too often I hear tweeter grain and harshness from compression at realistic levels. Another issue is resonance from light weight materials that ring like a "bell" - this can be very intrusive as it is completely unrelated to the music playing.

Tweeter compression and ringing is all too common and it is this which causes a loss of "smooth grain free silkiness" at realistic "live" levels, IMHO. It is one of the principle reasons that many high end systmes are "highly resolving but fatiguing" or have "etched" sound - and just don't sound natural in the way electrostats do.

In short, I do not see it as a high frequency extension issue at all. This is unfortunate for most people who tend to buy on "specs" and manufacturers that put too much money into high cost impressive extension tweeters instead of improving other areas of their designs. I am old school though - so feel free to diagree.

"I think the music should dictate the frequencies" Absolutely agree. I also agree that room and its contents would give you treble limitation. But say if you are 'designing' a dedicated room around your system what would you target the roll-off?

Per Shadorne it is the certain tweeter (silk dome, metal dome?)design that seem to be the problem/limitations and that electrostats do better in reproducing a true high freq response that does not sound hockey at live levels. How about ribbons? horn? Berrylium and the thingamjig?

Why can't the high end manufacturers finally solve this common basic problem then?

if live unamplified music is a reference, and the concert hall in particular, the attenuation in treble will vary with distance from the stage, the acoustics of the hall and the number of bodies in front and to the side of you.

in addition, the voicing you prefer should also be a factor. it is helpful to have access to an attenuation circuit, if you perceive too much energy in the treble region.

Real/live music extends well past 20K HZ. While some claim this is not audible I suspect it has an impact on the overall presentation. If you look at some of the finest components on the market, many exceed well past what has become the standard 20K range. While this alone may not put them in the absolute upper echelons of performers, it certainly is likely to contribute.

I agree completely with Mrtennis here and in experimenting through the years, especially with Quads, but also with other ELS, the only really satisfactory (to my ears of course) top end was found with the tweeter panels of the long defunct Servo Static speakers, which I tweaked, so I could attenuate them. Later with the Quad63s after much experimentation I settled with the aCapella plasmas which I placed on top of the Quads and drove them separately so that I could adjust their SPLs. They go up to 35khz. For the bats there of course, but the addition of these speakers had a tremendous influence (positive in the sense of more natural) on the entire sound stage and the rendition of transients, bloom,overtones and ambience .
Another thing is to be considered: Experiments have shown (unfortunatly I don't remember where I read about this, so I cannot point you to this, it was about 20 years ago), that listeners, unable to hear tones above 10khz, felt uncomfortable when music was played to them with its upper freqencies cut off above 18khz. So even if we cannot hear it, we obviously feel something is missing, when it is not there. So I personally am not a believer in fixed roll off points in the highs, but prefer, as MrTennis suggests, if I read him right, to set the highs myself, so that I have all the liveliness and ambience I wish for, without the sound per se getting thin, even if it practically means to set up a second chain to do so. (By careful placement, phase anomalies can be easily avoided)
At the moment, from what I've measured, the Sound Labs seem to start attenuating close to 20khz in my room and to me they sound just right. I still have the aCapella tweeters and will in due course experiment with them, driving them with the Zanden9600s to see what happens andI shall report, Nil, if you are interested.