Well, now that I have a new Denon 6.1 receiver, I can now add another surround speaker. BUT, I want to know WHICH speaker I can add. Does it necessarily have to be another center channel speaker? It was my understanding that 6.1 setups add a rear center channel, but can you just add another bookshelf speaker? I'm asking b/c for now I can add my old htib center speaker as the extra center, but down the line I'd like to upgrade all my surrounds (either JBL E10s or E20s for side surrounds), and was wondering it I'd have to shell out for another JBL center channel (not cheap!).