\rWhen you see an amplifier's RMS power out, you will usually see it rated at 4 ohms, 2 ohms, and sometimes 1 ohm if it is a subwoofer amplifier. This rating tells you how much power the amplifier will put out, per channel, when it sees a 4 ohm, 2 ohm, or 1 ohm load. You can change the impedance by wiring more than 1 speaker per channel and wiring them in either series or parallel. Most car speakers have single, 4 ohm voice coils and car subwoofer range from single 1 ohm voice coils up to dual 8 ohm voice coils. There are configurations in between, but that's a whole other story. If you are looking for an amplifier to power your speakers, the 4 ohm rating is typically what you want to look at. With Ohm's law, as the Ohm's decrease, the current increases. Since voltage remains constant, the power output doubles when the resistance is cut in half. This is not always the case due to ciruit design, but 100 watts at 4 ohms, is 200 watts at 2 ohms, and 400 watts at 1 ohm. Most full-range amplifiers that are designed to power speakers are not 1 ohm stable. Most are 2 ohm stable and bridgable. Most subwoofer amplifiers are 2 ohm stable, most Kicker and Rockford Fosgate amplifiers are 1 ohm stable.\rDo not just assume all amplifiers are bridgeable, 2 ohm stable, or 1 ohm stable. Double check the power output at the wired impedance to make sure you don't blow your amplifier.\r\r