Car amplifiers give you a clean power source that can drive your speakers without straining. Your music will sound cleaner and more defined at all volume levels when using an amplifier.
A subwoofer in your car will enhance your listening experience and adds an intensity to your music that smaller speakers just can't provide.
A subwoofers in sealed boxes tend to produce a tight bass sound. As oppose to a looser, boomier sound. It all depends on your taste of sound. If you do want that thundering sound You'll get this type of sound from a subwoofer mounted in a ported or bandpass enclosure.
