Sean is right, most decent speakers require amplifiers to reach their optimum performance. My analogy is this: Picture a 100 lb guy trying to lift 50 lbs. He may do it but he struggles, fights, and barely gets it off the floor. The same 50 lbs and a 350 steroid-induced muscle maniac. He can lift it with one hand and have the strength in his other to lift another 50 lbs or eat a powerbar or whatever else he wants.
In the car audio world his second hand is like an amplifiers headroom. You have more power to do other things like control your speakers movements accurately. The sounds you hear can be clean, sharp, and pleasant to the ears. Have you ever been in a buddies car and after listening to it loud for a long period of time and your ears feel sore, or want to turn it down on some tracks? This is listening fatigue. Your ears are sensitive towards certain frequencies and if they arent produced well in a car audio enviroment, your ears will let you know.
My suggestion, get the amplifier if you can afford it, or have credit with the store. the more power you can get to those speakers, the better they will sound and the money you spent on those nicer speakers will be well spent. This is especially true if you like to listen to music loud.
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