A buddy of mine chimed in on this. He competes mostly in SPL contests in Southern Maine / New England.
"There are tons of options that will be good for your application; Any decent brand with a high SNR ( Signal to Noise Ratio: I always look for 90db or higher, most low end equipment will be in the 60-80 range) and match the output to the drivers being used. Because you don't want distortion too much power is better then too little. He should be able to use the amp as an active crossover if there are tweeters. 6.5s/8/10in round speakers are the best option in most cases. Normally you see tweets in the pillars but to each there own on that one. Some newer cars are coming with 6x9 as a "subwoofer" :hahano: so less than ideal configs are everywhere. As far as American Bass, it's more "made in China" stuff, but all kidding aside they can make some good amps. Most of them have an unbelievably small footprint. I personally prefer (for a mild setup) and for 'cheaper' high-end gear JL, but every major player will have some decent stuff. I strongly encourage you to find one that matches the RMS of your speakers, and work with the space that you have. American Bass might not be the best, but for the price and size they are pretty darn good. Some amps will have better parts in them but it's more down to a per model vs. per manufacturer until you get into the high end stuff."