I've never heard of a class S amplifier don't you mean class D?
Why do you need a 0 to 30V, 0 to 3A PSU for an audio amplifier? I presume you want a general purpose bench power supply, rather than a dedicated audio amplifier PSU. A decent audio amplifier design shouldn't need a tightly regulated power supply. It should be designed to reject changes in the power supply voltage, without passing them through to the speakers.
The problem with a constant voltage and constant current PSU is Ohm's law states that it can't be in both modes simultaneously. When the load resistance is above a certain point which would exceed the output voltage setting, it will be in constant voltage mode, then when the load resistance drops below the point when the current limit would be exceeded, it will switch to constant voltage mode. The problem is switching from constant current to constant voltage modes and back with minimal oscillation and current/voltage over/undershoot. Oscillation can be avoided, but it's actually impossible to prevent over/undershoot.
Here's a simple constant VI regulator I lashed up in LTSpice. U1 needs to work with its inputs down to 0V and U2 with its inputs up to +V. V2 and V3 set the voltage and current limits respectively. D1 turns on when the regulator current limits, otherwise it's off. RL1/2/3 test the transient voltage and current response. RL1 applies a step load of 500mA, for 10ms. RL2 takes it into constant current mode and back again. RL3 as good as short circuits the power supply, when it's in the constant current mode.
C2, C2, R8, R9 and R10 form phase compensation networks which reduce the oscillation.
The transient response isn't great. Adding a capacitor across the output will cut down on the voltage over/undershot, at the expense of current overshoot and adding an inductor will improve the current mode, at the expense of the voltage mode.
