A high current amp is just that, an aplifier that is capable of producing a very large amount of currnet. When an amplifier puts out a specific voltage, the lower the impedence of the load is the higher the current required. So most high current amps would be stable at a low impedence, like 1 ohm per channel. A class X amp is similar to a class D amplifier (don't really know much about them). It is a digital type amp that is very efficient, but arguably would have some effect on the sound quality. As far as I know, the class of an aplifier (A, A/B, D, X, etc) has nothing to do directly with the amount of current it can produce.