Is that a trick question? No tranformer is used. Real audiophiles
know that transformers cause distortion. Therefore, the best
amplifiers are those that run directly from the power mains in an
AC/DC configuration. Some care in wiring is necessary to avoid
fatal electrocution, but that's a small price to pay for less distortion.
However, if you must use a transformer, all copper should be oxygen
free so that it has the lowest possible resistance.

Real audiophiles know no such thing.

To -- for a moment -- take a facetious answer seriously, wouldn't using
line-operated amplifiers cause problems with ground loops, especially when
connecting your system controller to speaker amplifiers at a distance? My
controller and power amps have balanced outputs and inputs, and there is
nary a hint of hum even though they are well-separated.

Just in case anyone takes the idea of direct powerline powering seriously...
Transformers permit almost any DC power-supply voltage. Without a
transformer, you're pretty much limited to DC voltages suitable for
thermionic devices.