These are very rare, and very special amps.
I first heard one circa 1980 when I was just getting into this hobby.
As I was in junior high school, the system was impossibly expensive and exotic - an Apt Holman preamp, Quatre Gain cell, Dahlquist speakers and a Thorens turntable with an Infinity black widow arm.
I searched forever to find one in mint condition for my collection, but havent really used it.
For future reference, it would be great to know who can service and/or upgrade them.