It sounds like you should have at least 30 watts per channel or so.

I would go with the highest voltage I can, because it takes at least 40 volts from the power supply to deliver that much power to 8 ohm speakers.

40 volts DC will limit you to a 40 volt peak-to-peak maximum output waveform, which is 14 Volts RMS.

14 volts RMS applied to an 8 ohm speaker only produces 24 watts of power, so that would be your maximum peak output power with a 40 volt DC supply.

Large automotive amplifiers use a DC-to-DC converter to convert 13 volts to + and - 40 volts or more to get an effective 80 volt or higher power supply, which then can give a lot more power.