The amplifier would ideally have a 12 volt trigger that would power it up -after- all of the power-up transients from the pre-amp and sources have come and gone. Likewise the amp should be the first thing powered off. A soft start circuit is not advisable because ideally, the amplifier should handle that on its own. Power amplifiers that are well designed often include a mute circuit that cuts drive to the output stage until the rest of the amplifier itself is powered up and stabilized.
