RT800i's now my mains, RT600's moved to side surround. I'm still using the RM6752 center channel because I'm going to have to get creative about where to put the CS400, it's
massive. Was up until 2 on Tuesday night rewiring, putting banana clips on all the cables, and listening, so far with the Yamaha YPAO settings, except of course I reset all the speakers to small when it detected the four towers as large. Only 2-channel listening - mostly to DirecTV music channels, so far at very low volumes, but there is a major improvement in power and clarity from the 600's to the 800i's. I can hear details in the music clearly at -40 or -45 dB that I could not hear with the 600's until I cranked it up to -25 or -20. Although "50 Ways To Leave Your Lover" did sound a little peculiar until I realized the AVR was still set on "Hall in Vienna."
No real feel yet for improvements in the soundstage and imaging, I haven't really fiddled with toe-in angles yet, or had the opportunity to listen at normal volumes during daylight hours. But so far I'm really pleased.
I have to give some thought to where and how to put the CS400. Furniture space constraints are annoying. I probably need to build a shelf which will rest on the high window sill behind the TV with braces down to the top of the entertainment unit behind the TV. Plus: the speaker will already be angled down towards listening position a bit if I use the window sill like that. Minus: we'll need to figure out where else we can put the Xbox Kinect camera which is clipped to the top of the TV at present. I could reconfigure the component shelves and arrangements and put it in the unit, but it will be hard to angle up and I'm not sure what being surrounded closely will do to the sound, even if I cut out the back of the unit behind the speaker.
If this winds up insurmountable, I may sell the CS400 and get a smaller center.

We just bought the furniture, so there's no replacing that for a while.
Thanks to all that have helped me figure out what I was doing! Esp RobLee, Stanley, Ron Temple. And, really looking forward to hearing what Stanley has to say about the LSiM703's when they arrive. Onwards now to a better sub, sometime this summer...