A sub can effect the performance of 6x9's. They say you should box them in to prevent that. Others will just say take the 6x9's out if you've got a sub.
£20-£30?? That's way too expensive. I can make them out of high density board and padding to dampen resonance for a lot less than that. It's not difficult to build them and the sound will be aimed in your direction rather than reflected off the back window. Also gives you a bit more boot space