Well, it is not exactly true. The way modern power supplies work is to
rectify AC to DC, modulate it with high frequency (1KHz or so) with some
transistors to feed a small transformer (the higher frequency, the smaller
core you need), and then rectify it again.
The power loss on transistors working in saturated modes is mostly due the
voltage loss (which is constant determined by physics of the PN-junction
or field-effect) multiplied by the current. With lower voltage you need
higher current to get the same power, so the power dissipation on switching
transistors is higher.
Therefore, high-voltage power supplies are more efficient than low-voltage.
The reason why sync mux racks are quiet is because they don't do any
fancy high-speed stuff (after all, those muxes are nothing more than
glorified shift registers) and so they don't need any power-hungry