There are many factors incorporated to make a good sound from an amp.
The two points you mentioned.
Balanced circuit = Ground free circuit = reducing noise
Powerful Primary Transformer = Capable of giving enough current.
A part from this it's important to use high quality components, caps, resistors and Power Transistors. The latter were in many cases especially made for Sansui by Sanken such as NMA/NMC.
The design of circuit is of course another very important factor. Thank's to Mr Ohshima and Mr Ohtsuka.