The ac transformer is simply the origin of the audio quality
A low quality transformer can reduce the audio quality. You can't reach the "silence" and fill your room with the smaller details, if everything is covered by a hum noise generating transformer.
It is the most expensive electric part, and second only to the cabinet.
Traditional EI AC Transformer
It is the cheaper kind of ac transformers and it is very common. Potentially is one of the worst sounding.
Resined Traditional Ac Transformer
This is a classic ac transformer encapsuled into resin to avoid noise.
Toroidal AC transformer
It is a donut-shaped transformer, and it has more efficient(loss less power) than the classical EI transformer. The noise produced is much lower than EI transformer. Obviously it costs more.
It is the best type of the traditional ac transformers. It is the most efficient, silent and has explosive power. If a Brand used it in a product, is showing you that they looked for the maximum audio quality. It is the most expensive type, so you can find it in high quality hifi, dvd and blu ray players, and rarely into an amplifier.
Switching Ac Circuit
The switching ac module is very common in video players, because is much lighter, smaller and cheaper than the one made with the classic transformer. It has very good power performance, so it can be made with smaller size compared with classic ac power made with tha ac transformer and big capacitors. Has one only big problem: it generates high frequency noise.