Adding subwoofer to amp set up?

A separate amp is needed. Mono block amps were made for subs, multi channel amps were not. But they can work with subs though, even though they are not meant for them. A mono block is also going to cost less money as well. The only way that you will be able to connect a sub to that amp is only if it is a 5 channel amp, the 5th channel is dedicated to subs.