I feel you are robbing yourself if you don't choose the 685 or 683. Not a fan of the 684s or 686s, for the small price difference in the larger speakers in the 600 series. The center is alright, I watched Dark Knight on it and the voices I felt sounded squelched.
90 out of 100 on the Denon reciever is way too loud. When you choose the other volume display option you generally shouldn't go past 0db.
You can get a better subwoofer for $550 right in Anaheim. Pick up a http://www.hsuresearch.com/products/vtf-2mk4.html