What is the ideal High Freq extension?

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.

Detlof,

Sure I would certainly be interested to know the your acaoell tweeter experiment. How did you measure your Sound lab extension to 20K hz? Via Sound meter? with necessary correction applied? ( Here in US we have Radioshack sound meter that I typically use).

CKoffend, thanks for reminding me that live music extends well past 20 Khz (How can i forget? with recent threads comparing analog vs digital medium) as much as like 70k hz and beyond for certain instruments.

So in a perfect system you would want 'flat' response to and beyond 20khz- No roll off. Hmmm Now It makes sense why some audiophiles(Detlof and some other I know)prefer super tweeters hooked up to their main speakers.

Now that I think and look back I do remember auditioning a super tweeters in an all audio note system and discerning difference between these tweeters being on and off!

Any recommendation which super tweeters to try with my Dunlavy Vs?

It depends on how old you are and how good your hearing is. When I was in my late teens, I could hear up to 18Khz. Now, 30 some years later, I could hear no more than maybe 12 or 13Khz and music sounded very different to me than when I was much younger.

I have heard speakers with purposed response to 50Khz.While you might not hear that high up,the over-tone structure may be more completely revealed.A lot has to do with the crossover and phase linearity.
You must remember when digital "brick-walled" everything above 20Khz.Thank goodness we got past that!

Nil, I borrowed a hand held spectral analyser with a microphone built in its "nose" cum pink noise generator in a wall wart type of thing. Can't remember the name of it, but it was US-made, if I recall right, had a lot of LEDs in its display. A nifty little thing.