What sounds best will be different for every car. Try firing the sub in both directions and see what works for you. It all depends on where you will end up with the most interference, nodes, and what not.
Either way you will probably find removing the 6x9's beneficial. Making a good passive radiator setup is more complicated than this.
Either way you will probably find removing the 6x9's beneficial. Making a good passive radiator setup is more complicated than this.