What is the ideal High Freq extension?

I think the music should dictate the frequencies - not your own judgement. In this case, the extension from the speakers should always be higher than that of the recordings. For good transient control, you need high system bandwidth - the higher the better.

But the thing you are forgetting is the room. It ultimately will dictate the extension regardless of what you think it should be or what your speakers are rated at. Typical in-room measurements using an averaging technique will show that most rooms start to roll off the treble around 9kHz (like my room). The amount of furniture and the texture of the materials will change it up or down - but generally down is much more likely.

Arthur

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.