Yes Lawrence, they are, and are still around, second hand, as are on occasions, various ESS models of speaker.
They didn't do any further work on the Heil after '74, except to use the 'wedding veil' gauze across the diaphragm peaks to aid control. A shame really. The speakers are still made.
When I had my first pair I acquired the brochure for the power amp, in about '95, and it was so far ahead in spec I was stunned.