Importance Of Power Supply : Transformer Ratings

It is to my impression that the power supply plays a vital role as the source of energy for power amplifier selection. May I know how does the power supply contribute to the quality and/or sonics of the amp?

How does a 500VA power transformer hold an advantage over a 400VA power transformer, and how do we deduce the power transformer ratings of amplifiers if the figures are not shown in the user manual or specifications ?

Any advice would be most appreciated.