Power supply for Audio amplifier : high-voltage based watts or high-current based watts?
A class D amplifier is basically a fancy bipolar buck converter, which means it can output any voltage into the speakers up to the power supply voltage.

The amplifier is rated for an input from 8v to 24v. It will function down to 8v, and provide maximum power at 24v. 

The absolute maximum power you will get into a 6ohm load, assuming bridged speakers and neglecting all losses, is V^2/R, or 24 watts at 12v, and 96 watts at 24v. Clearly you will need the 24v supply to get 40 or 50 watts.

As a class D is a buck converter, it will draw less current on its input than it delivers to the loudspeakers (not magic, just conservation of energy, and the reason class D is popular), so the lower current 24v supply will be fine.