The Carver magnetic amplifier, "mag amp" is not the accepted definition of a magnetic amplifier. Traditional magamps were used for a variety of industrial and military applications. As I recall, a mag amp is a line voltage conditioner that keeps the line voltage fed to a load at a constant voltage (120 volts) despite a wide variation in input voltage. It does this by first boosting the line voltage with an auto transformer and then reducing the voltage as needed with a DC bias on a bucking transformer.
As I recall, the Carver concept varied the DC bus voltage to an audio amplifier to minimize heat dissipation in the output stage. This reducced the size of the traditional input transformer and heatsinsks. If it was a revolutionary design more people would have followed suit. To my knowledge, none did. Below is a link to a company who makes magamps. The adventurous person will do a search on magamps and find some interesting applications.
CPC Magamp