Amplifiers
What is the benefit of a head unit with a higher voltage preamp output?
A piece of equipment that has a higher maximum output voltage is not necessarily going to sound better than one which is only capable of 2 volts output. The higher output will allow you to reduce the gains on your amp (or any down line signal processor) which will lower the noise floor of your system. If you are not having trouble with noise (alternator, hiss...), you may not benefit from the extra output voltage. You also need to realize that the voltage may be more than your amplifiers can handle. The extra voltage will not damage the amplifier but if the lowest sensitivity (gain setting) on your amp is 2 volts, and you drive it with anything more than 2 volts, it will cause your amp to clip*.
*This assumes that the gain setting is the voltage that will drive the amp to max power. Some amps are underrated and may have slightly more headroom and therefore will clip at a little higher power. Why does one 1000 Watt amp $850 and another only $99? Are they both the same strength?
A great way to get an idea what an amp can do is to look at the size of the fuse in it. Here's where we refer to Watt's Law (Watts = Voltage X Current [Amps]). For example, if an amp has a 15 Amp fuse, running off a vehicle's 12-14 volts of power, it could produce a maximum of 180-210 watts of workable power if it's a well designed amp. SO if you look at that $99 1000 Watt amp and it has a 10 Amp fuse in it, guess what.
Wattage Ratings on amps can be extremely misleading. Unfortunately, the industry isn't very regulated when it comes to power ratings. Currently in car audio, the most commonly referenced types of Wattage ratings are Continuous (RMS) Power and Peak Power. Peak Power is the rating most used by low cost amp manufacturers to make an inexpensive amp appear to be as powerful as a more costly one. Peak Power measures the largest signal the amp is capable of producing. The rules for measuring Peak Power are, simply put, "No Rules". However they can get that piece of equipment to spit out a signal and get a measurement, that's OK. If they want to give it 50 Volts and measure it right before it explodes, that's OK. More credible manufacturers, like Rockford Fosgate, Alpine and Boston Acoustics, will measure Peak power under conditions that can exist in a real car.
Continuous Power is a bit more realistic. Generally, Continuous Power measures the amps under these conditions. A supply voltage of between 12 and 14.4 Volts - A 20 to 20K Hz signal input signal - Measure the amplitude of the largest signal it can produce UNCLIPPED (see Clipping in the Audio Glossary). However, there are still manufacturers out there that will bend these conditions to achieve bigger ratings by measuring the output at the frequency the amp is strongest at, not 20 to 20K Hz. For years, if you wanted to figure out what to spend on an amp, it was pretty much a dollar a watt. That figure has improved slightly with the advanced technologies of today, but not by a huge amount. Price will still tell you much more about amp capabilities than wattage ratings can.
The industry is currently working on implementing a new standard for measuring and publishing amplifier specifications called CEA-2006. The goal of this rating system is to create a level playing field for comparing ratings on products. There will be two primary ratings for amps - "Power Output with Total Harmonic Distortion plus Noise" (indicates the actual continuous power developed by the amplifier, into a 4 ohm load, over the entire design bandwidth, while producing not greater than 1.0% THD+N) and "Signal to Noise Ratio - A weighted" (indicates the amount of audible noise inherent in the product at 2Vrms ). 
A common misconception is that your speakers are unable to handle the power your amplifier produces. The distortion is actually caused by lack of power. Any power amplifier has limitations. When it's driven past these limits, it causes something called “clipping” in the amp which makes the sound distorted.
Clipping is a condition caused when an amp essentially “runs out of room” to produce a large musical signal. As volume is turned up, these signals get increasingly larger. Think of it like jumping on a trampoline indoors, with the ceiling of the room representing the limits on your amplifier and how high you jump representing a musical signal. The more power your amp has, the higher the ceiling is. If you have a low ceiling, you won't be able to jump very high before hitting your head. If you hit your head hard enough, everything distorts. That's basically what happens when an amp goes into clipping. Clipping can damage speakers very easily by producing excessive heat on the voice coils.
The type of music you listen to will affect just how fast you reach this point. The more bass the music has, the less you have to turn up your receiver before it distorts. Lower frequencies require more current from your amplifier, causing it to exceed it limits quicker. This distortion can easily damage your speakers. So whenever to start to hear it happen (irregardless of where you're at on your unit's volume scale), just back off your volume until it stops plus a little more. This will keep your speakers from being harmed.
Woofers
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