Before ESS introduced the Heil AMT, they already had a line of really good speakers using KEF drivers. The way Dr. Heil and ESS hooked up was that the Doctor came into the ESS dealer in San Jose (TV-Audio Center, Inc. I was a customer of theirs.) to see about getting a pair of KEF B-139 woofers (the 9" X 12" flat-faced oval driver you may have seen pictures of) to use in the speaker he was developing (the AMT, of course.). ESS was using the B-139 in their Transtatic model speaker (Dave Wilson used it as well, in his WAMM loudspeaker. It's a REALLY good woofer.), along with a KEF Bextrene 5" midrange driver and a trio of RTR electrostatic tweeters. The Transtatic was actually a much better speaker than the Heil AMT, but cost $1200 in 1971 dollars. It was competition to the Infinity Servo-Static loudspeaker. I found a pair in the early '80's, one of them having a replacement woofer. I called ESS in Sacramento and got their last remaining B-139! Still have the speakers.