hifi engine

Sansui AU-9900A vs AU-9900 whats the real difference? They sound very different?

Hello. thank you reading this message. I came in possession of an au-9900A in addition to my au-9900. They sound very different? au-9900A much more dinamic and the volume control seems more sensitive. Is much earlier on hard volume. While I really have to turn the volume knob of my au-9900 before some power is really released. What is the real difference between the AU-A models and the first series of AU models. Does anyone have an idea? thank you very much and greetings Dutch Sansui fan Bart!