The purpose of a sub is not to give you the boom boom boom you hear coming from lowered civics and lancers, the purpose of a sub is to give you nice clean crisp bass that speakers cannot provide. When a sub is set correctly you shouldn't be able to hear which direction it is coming from, I really do think you need a sub.

If you get a sub, then your speakers can focus on giving you the sounds that they can do best.

Really suggest you give FHRX a call, im sure he can give you a nice setup, he is known to be one of the best installers in NSW.