They are no longer in business. What else do you want to know?

ESS mated their high frequency AMT unit with a conventional dynamic bass driver. The AMT is so sensitive that it overpowers the woofer. However the sound from the AMT should be low distortion, based upon its efficiency and the horn loading of the diaphragm.

ESS later developed a "full range" AMT, which I heard once, but it never sold very well as a speaker. It was a high-frequency AMT paired with a low frequency AMT. The bass unit was driven vertically with carbon fiber rods attached to plastic (Lexan?) manifolds that pump the air in and out horizontally. Interesting idea though.

I thought the speakers sounded fine for those days, late '70s.

Not sure if anyone is using this AMT driver now. There was some articles in the DIY mags (the then Speaker Builder, or Audio Amateur, which has a new name now) on how to make an AMT driver. A friend and I (in grad school) wanted to try this but never got around to it.

I had a pair of ESS amt-1b's back in the late 70's. They were very open and airy and had good bass, too. Not the hot ticket for high spl's, though. I destroyed two of the amt diaphragms cranking zee rock und roll.

I think the patents are expired now. I know some other companies are using the technology. I don't know of any here, though some of the European designs may be available.

In 1972-ish, I heard a demo of a "quadrophonic" system, where the front speakers were ESS Heil AMT 1a's. The rear speakers were Bose 901's. It was cool and at the time I suppose it was the best/highest end system I'd ever heard. Can't say that I have any useful memory of what it really sounded like, I just knew it was way better than my RCA clock/radio.