if you feel the sound loud and clear enough then no need amp lar. if you want the low bass sound the you can get a 2 channel amp for powering the sub only.

you can also power 4 speaker with 2 channel amp, but you cant adjust the volume between front and back speaker. this is how to do it: from amp left positive to front left speaker positive, then same speaker negative to back left speaker positive, then back speaker negative to amp left negative. this way you produce 8omh, which sounds lower but better than 2 speaker joint to same positive and negative, which result in 2omh for the amp, chapalang amp sure cannot tahan one.