The T-77 is the top of the Chapman line; the T-6MkII is likely now above the older T-7 model. From what I gathered talking with Mike F, he and Stuart prefer the sound of the T-6MkII to the T-7.
Being halfway across the continent and never hearing the T-77's prior to purchase, - I did own two pairs of T-7's, I can't comment on comparison directly of the T-6MkII and the T-77's. Mike (and of course Stuart) have heard both on the Chapman Audio Systems rig.
I'm sure Mike will post a review of his brand spankin' new T-6MkII's when he gets a moment. I have posted review of the T-77's on the audioasylum.com
Your best way of ascertaining the differences would be to call Stuart directly; he could tell you precisely, technically as well as experientially, what the differences are. 206-463-3008 Leave him a detailed message, if necessary. He can also discuss surround/center speakers. He does custom installs in homes, but I'm not familiar with any surrounds/centers. They may exist, but I've been focused solely on two channel.