Yes, there are different kinds of amps. Class D mono blocks are for subs, and plays the low frequencies. Class AB is for mids and highs, and plays the full range of frequencies. You can use a Class AB for subs. Most of them have built in cross overs for high pass and low pass. There is also Class A and Class B. Both of them are not really good.
Class A is not efficient enough to use, Class B is crap. Class AB is a compromise of the two, by being both. It is not as efficient as Class D though.