I previously owned one of these amplifiers...
As you mentioned, it was a rare bird. Most people confuse it with a solid state product Dynaco produced of the same name. The Stereo 80 addressed the well-known complaints of the venerable Stereo 70: a bigger power supply and a "proper" driver (Mullard long tail pair vs. traditional Dynaco Concertina) stage. There were also a couple of niceties such as the triode/UL switch and bias LEDs.
As you mentioned, it was a rare bird. Most people confuse it with a solid state product Dynaco produced of the same name. The Stereo 80 addressed the well-known complaints of the venerable Stereo 70: a bigger power supply and a "proper" driver (Mullard long tail pair vs. traditional Dynaco Concertina) stage. There were also a couple of niceties such as the triode/UL switch and bias LEDs.