Most amplifier today are build-in with an internal crossovers or additional signal processing. Signal processing comes in the variety of simple high-pass or low-pass crossovers to full adjustable digital crossovers with time alignment and 31 band equalizers.
If you plan the amp for powering midrange speakers and tweeters, make sure the high-pass crossover has the frequency adjustment range perfect for the application.
If you plan a three way system with separate amp for tweeter, midrange speaker and a woofer, you have to make sure the crossover range meats the recommended frequency range for the speakers.
As for sub amps, an amp with a bass boost or even better with a subsonic filter, a subsonic filter is a circuit that protects subs from excessive low-frequency excursion, make sure the amp you choose has at least the electronic crossovers that you require to normally control the system.
Additional control and adjustments can create a more flexible and also can provide a better sound system.
