A DJ mixer is basicaly nothing but a pre amp.
But I'd say it's only ones made by Urei, Allen & Heath or Pioneer that would be anything approaching half decent Hi-Fi sound quality.
Other than that though, yes, it's definatly better to just use a power amp on the end of a mixer rather than an integrated type of amp. Otherwise you'd just be plugging one pre-amp staight into another.
Just remember though; That mixers harmless looking little master volume knob suddenly becomes very capable of popping your speakers in the wrong hands of those that might not be aware!