Disclaimer: This extremely rare item is owned by my good friend, vAsurada, who lives in South Korea. This article would have never been publicized had he not given me permission to do so. My infinite gratitude goes to him.
As previously explained, STAX team up with the scientists at IRT(Institut fur Rundfunktechnik) to further enhance the sound quality of their products. And the ultimate result is ED-1, a diffuse-field simulator for SR-Λ Pro headphone released in 1988.