You would be better off purchasing another battery than you would be with a capacitor.
But to answer your question, a capacitor is NOT necessary, provided you have a proper vehicle electrical system. In all honesty, you can try your subwoofer amplifier first and if you notice excessive dimming or that your vehicle won't start, then you can look towards adding another battery or upgrading your alternator.
The only good I have seen a capacitor perform was to filter out AC ripple from an alternator that was going bad. It also helped in some imports that had Electrical Load Detectors by causing the charging system to see a load therefore keeping the charging voltage constant versus dropping to 12.5 volts after the charging system leveled itself out.