hooha,
I respectfully disagree. I doubt whether many people could hear above 20Khz. Despite what the SACD/DVD-A marketeers are trying to sell me, I listen to what the audiologists and physiologists have taught me: I am a young fella in my mid twenties and even at this age high frequency hearing loss has already occurred.
These days many tweeters have extension out to 30Khz or beyond. eg. ScanSpeak D2905/9x00 series. I tried to test my upper frequency hearing limit by playing a 25Khz tone at +10dB relative to my normal listening level. Perhaps you could try it and see if you can hear it?
IMHO too much emphasis is placed on the ultrasonic region when people think of "harshness". People see the big sharp ultrasonic spike and get scared off. But in relative terms what is happening below 10Khz is more important than what is happening between 10-20Khz, which is even more important to what is happening above 20Khz.
Sometimes one of the first things to investigate in speakers that are described as harsh is the crossover region, or whether there is sufficient baffle step compensation. I have heard several crossover designers comment that when fine-tuning filters, a significant difference in tonal balance can be heard with a change of as little as +/- 0.5dB in the 100Hz to 4Khz region. Of course, this is strongly related to the region where the ear is most sensitive. But above 10Khz these changes make a difference not to tonal balance or perceived treble, but to perceived "air", or "presence"
The 27TBFCG is extremly smooth under 20Khz. Subjectively the 27TBFCG is one of the smoothest softest tweeters I have ever heard. Objectively it is one of the best measuring tweeters available to DIYers, irrespective of price.
Sorry for my little rant, but it's all in the filter folks.