If you can find one, buy a Cerwin Vega EXL-400. It is 50 watts x 4 channels into 4 ohm speakers. This amp is based on some high end designs by Robert Zeff, founder of Zapco and Arc Audio. Cerwin Vega could not sell them and dumped their entire stock on the surplus market in 2008. You can find them on Ebay and the car audio forums. A lot of members on the forums bought multiples of these amps for 60-80 each when they were originally for sale just to stock up on them. I just bought one for $115.00. Still a very good deal. You won't find anything close to this quality and performance for the price.
There is also a 2 channel model, the EXL-350. Also recommended.
If you want to know more, go to DIYMA and search for posts about these amps.