Some power amps are designed with a supply per output device, but others (like the A1400-8) aren't. There's nothing inherently wrong with a design based on a single power supply. But more often than not, the single supply isn't big enough, and dividing it among several output devices isn't ideal. But then some multi-channel amps, use one power supply per channel, but in an effort to squeeze that many transformers in one chassis, they are all under-sized.
It's all about the over-all design, and there are many ways to do it wrong.
