Upgrade speakers or add a sub

So I have an aftermarket head unit with rca outs. The previous owner had a sub because the wires are already ran and the previous owner ran wires new wires to the door speakers to the head unit because I had the infenity system. The whole thing is kinda butcherd. So my question is would it be better to add a 4 channel amp and upgrade the door speakers or just add a sub, I was thinking 8" to save on space but could possibly do 10"