Lordathestrings" advice is probaly the best advice you can get and and the only points I would add to it are, Some shops are better than others, they'll take the time to check amps out and will know something about it when you start asking questions, Try to find someone as knowledgable as possible and take them with you, You have already started doing the most important thing and that is educating your self , buying used amps or new ones for that matter is something you want to do with your eyes and ears wide open, and with as much knowledge of the product as you can ARM yourself with, Good LUCK, and Good Hunting!