[mikegett]

Quality of amplifiers have increased drasticaly over the past ten years. Just about any brand will have at least one good line of amps. The important things to look for is how you will wire the amp , and how much wattage will be needed. Amps will produce less distortion when wired at a higher ohm. I try to keep mine at a four ohm rating. However, it can be cheaper to buy a amp that can be lowered to 2 ohms with the same wattage. If you go below 2 ohms, stick to a class D and buy a very high quality. Otherwise the distortion tends to be excessive. I have used Pioneer, Sony, California, Nakamichi, Eclipse (I work for them), alpine, and A/D/S. I currently only have eclipse in the car. Not bad, but they are similar to Nakamichi. They tend to be directed more toward mids and highs for sound quality and lack the punch in lower frequencies. The pioneer, sony, and alpine were decent. The only one I hated was California. That amp was way over rated and filled with distortion. If you realy want a great amp then go for A/D/S. They are probably the most underrated amp I have owned. The sound quality is unmatched. It is hard to find a amp that can do mids, lows, and highs without coloring. A/D/S has nailed it. They are expensive but well worth every penny.
One other thing to concider is what Head Unit you use. What is the preamp rating? Many HU's offer four to eight volts on the preamp. Check out the amplifiers input voltage. You may need to look into the manuals. Sometimes it is printed around the gain knob. If you overpower the amp then noise (a hissing) can occur. After awhile the amps inputs will fry. Most newer amps will except up to eight volts, but if you buy from ebay it could be a older model.