I've done some searching and apparently "pistonic" is a made-up word in the audio world, and the original use was when multiple coils and magnets are used to drive the diaphragm more evenly, like those rectangular and odd-shaped speaker drivers of the '70s (many by Yamaha).
What I really meant was what we're used to hearing from some metal domed tweeters, which invariably have a resonant frequency spike (just about the range of hearing) and what's called "oil canning" which is also a distortion characteristic of overdriven metal domes.
My main point is that the accordion pleat compression drive of the Heil-type motion transformer, with its larger radiating surface and minimal motion, doesn't exhibit the harshness characteristic of many metal domes, and therefore might sound rolled off when it is actually delivering accurate treble without accompanying mechanical distortions we're used to hearing.
What I really meant was what we're used to hearing from some metal domed tweeters, which invariably have a resonant frequency spike (just about the range of hearing) and what's called "oil canning" which is also a distortion characteristic of overdriven metal domes.
My main point is that the accordion pleat compression drive of the Heil-type motion transformer, with its larger radiating surface and minimal motion, doesn't exhibit the harshness characteristic of many metal domes, and therefore might sound rolled off when it is actually delivering accurate treble without accompanying mechanical distortions we're used to hearing.