MOSFETs have some very good properties for audio amps. Lateral MOSFETs are more thermally stable than BJTs thanks to their negative thermal coefficient. They are easier to design with too - no beta droop, no secondary breakdown, automatic current limiting, and are voltage driven. However BJTs have a much higher transconductance and thus higher linearity so they are not to be totally dismissed. Linearity is after all one of the most important properties we want and look for in audio amplification. In the (near) future however, there will be some lateral MOSFETs that will rival the best BJTs in terms of linearity, at which point I will go with a MOSFET amp.
JFETs are quite different and are not used much, if at all, in output stages. However they have excellent input impedance and low drift so are more suitable for input stages and current sourcing.
Balanced connection has nothing to do with transistors so it is definately not the only way to go.
Hope this helps. Arthur
