You cannot use 70V line *step up* trannies with domestic
hi-fi amps. Line voltage power amplifiers are specially
designed to drive the included step up transformer --
which presents a lethal load to most SS amplifiers.

You might be right... And I'd certainly be cautious about using a
constant-voltage transformer with an amp not specifically designed for it.
But I suspect you don't understand how a transformer works -- that is, how
it transforms impedances.

Let's say a transformer is rated a 4 ohms to, oh, 1600 ohms. One side //does
not// have an impedance of 4 ohms, or the other side 1600 ohms. Rather, the
transformer has a turns ratio of 1:20, which produces a impedance
transformation of the square of the ratio, or 1:400.

The transformer would work in //any// application where this impedance ratio
was needed, whatever the specific impedances were -- for example, 100 to
40000 ohms. (I'm ignoring considerations such as primary inductance,
leakage, etc. The principle is the same.)