It has always puzzled me that adding a super-tweeter always seems to add "air" to a soundstage even for people with hearing that doesn't get past 12Khz. I once posed this to Dick Sequerra and his response was that you can't assume that the ears are the only inlet to the brain for sonic information. He went on to say that the ocular cavities are a major entry point for high frequency info. I thought about this for a while and then filed it away to some deep dark corner of my brain to digest at a later date.
Recently I came across this article
that puts forth a plausible explanation into this and seems to tie in with other reports of listener fatigue from systems where the "highs don't sound right"
I have also always noted the recommendation for eye protection when using ultrasonic cleaners and always thought is was just a "belt and suspenders" safety thing for possible splashes. This opens up a whole new concern given the popularity of ultrasonic cleaners for vinyl.