In past few posts, I've been describing the "more defined bass" effect that seems to result from using a supertweeter. I did not invent this idea, you can find it in many other blogs.
And there is another claimed effect from supertweeters. It is claimed they do not make the sound more harsh. That is the weak form of the claim. The strong form is that they actually reduce harshness.
If you look into the claims made for supertweeters, these and many more are common.
Others scratch their head with incredulity, and say that they can't hear as high as their current tweeter can play, so why do they need something that plays higher? Others say they may get too many highs, or at least too much stridency and harshness, as it is, so why would they ever want more (and apparently they don't believe the claim that supertweeters do not increase and may decrease apparent harshness).
Many of these skeptics have not even heard any supertweeter. And since most supertweeter setups are consumer addons (though I'm just guessing, it could be the other way around), or even otherwise, it can always be claimed that the supertweeter setups they have heard were not "properly integrated". Supertweeter integration is a large topic that you could read about endlessly.
On the other hand, I'm not aware of any blind tests, and it may not even be possible in many cases to do a blind test. It is likely beyond the means of most hobbyists.
Well I never say my tests are ultimate and definitive, but I continue to sense the supertweeter as decreasing harshness, though sometimes adding a distinct unwanted sonic signature or localization.
Last night I did another test of the +10dB and +15dB crossover levels for the supertweeter (at the current 15.5khz crossover setting). Listening to "Lucky Man" from Emerson Lake & Palmer, I was thinking Greg Lake sounded a bit strident. (A lot of that, no doubt, is the recording, intended to give Greg an otherworldly sound.) So I then changed to the +10dB level. No particular change in Greg Lake's vocals, but now the highs seemed more chaotic from all over the soundfield, there were lots of distractions in the high frequency range. Restoring the +15dB level restored sanity, though perhaps you could claim a kind of totalitarian simplicity, with Lake's voice commanding a swirling mix of instruments.
I have now read some claim the Elac 4pi tweeters can sound metallic if played too loud, and I am struggling with that. To get the most good effects from the supertweeter, it has to be played loud enough. Perhaps to get any good effects... But then, at times the supertweeter can give itself away. The supertweeter can be "hidden" completely by making it's level low enough. But then it doesn't seem to have the good effects. So there is a fine line between not-loud-enough and too-loud, and sometimes one can wonder if there's a line at all.
I think there is, unfortunately, a tendency toward metallic sound from the Elac. If the Elacs were perfect, like the pulsating plasma in Hill Plasmatronics, they might not have that sound. The sound can possibly be tweaked with DSP, class A amplifier, cables, etc., etc. But it complicates the integration problem.
Speaking of Plasmatronics, I had a friend who was combining the plasma driver with Acoustats back in the late 1970's. It sounded great then. But I don't consider the Plasmatronic driver to be particularly practical and I'm not even sure I'd want two in my living room.