SOZ advice

Can anyone comment on why deeper (higher voltage) rails produce a sonically superior amplifier (all other things equal). Is this just because for a given bias resistor a higher dV across it will produce higher quiscent current at the drain of the mosfet (picture the SOZ).

Grey commented in another post (SOZ biasing) about using non symmetrical rails to reduce the heat dissipation of this amp. What are the requirements of such a setup if I were to use a resistor and not a current source as suggested by grey?