The follow-up to the Threshold 800, a not-easy-to-find big amp that introduced Nelson Pass as a coming star in the field of audio. What made it an interesting product, aside from its unusually smooth sonics, was the fact that it was called a Class A (high-powered) unit. About which there was some controversy, particularly from other designers, who thought Pass had pulled a sleight of hand. The trick here lay in the biasing of the unit—one then unconventional in the extreme. The use of shifting bias allowed the unit to stay in Class A as the demands on its power output shifted. The 400 would go into Class AB if the sonic demands exceeded its capabilities (said to be a rarity). What interested me then was just how closely it sounded like Class A, but with considerable power up to a quite loud point, it had most of the advantages of Class A sound, with few flaws (the unconvincing bass, lack of sparkle in the upper midrange) as well as a tendency to oscillate and blow fuses.