I have at present a Mission Cinema 7, 5 speaker setup with a kef sub which works great. At some point in the future I am thinking of making the system 6.1. I have obviously already got sub, front left and right, centre, and a pair of dipole rears. What type of speaker do I need for centre rear? Do I get another centre or do I need a surround? I was also thinking of getting a pair of centre rears so I'm ready for 7.1 in the future.