Once again thank you. The Stereophile article is really thorough on the Sound Lab load characteristics. I am not well versed in engineering, physics, and associated acoustical metrics: so, it will likely take me a couple of months to really understand the article.
I visited less than a handful of audio shops over the past 15 years that had Sound Labs(two with a pair of A-1's and another with a pair of U-1's) on hand to audition: none had amplification that was able to successfully drive the Sound Labs (They complained that there was some fluttering noise/distortion in the diaphragm or that the amps' overload lights were triggered even at low volumes: They didn't want to sacrifice the amplifiers, as they expressed, and Sound Labs sadly sat there unplugged).
Our Sound Labs just came in a little over a week ago: The S/500 opened up a new acoustical dimension by bringing them to life in a way that I was never able to experience before. Piano reproduction -- attack and decay -- and the immediacy(speed?) of instruments like cymbals, hi-hats, is simply uncanny.