You can add more speakers for better sound quality. However, make sure you have external amp for them. HU will not have enough power nor the ohms for additional speakers. Let's say you have 2 pairs of speakers for the front and back. To add 2 more to the front, connect each speaker in parallel to the existing ones (pos to pos and neg to neg). This will make each 2 pair 2ohms. Now have an amp that's 2ohms stable.