I hate sounding like a nit-picking bastard because it looks like you have the makings of an awesome theater system goin' on... but...
Since you are doing an AT screen... you really should use the exact same
vertically aligned tower for your center as your left and right fronts. Horizontal center speakers are a design and sonic compromise in order to place the center above or below a normal TV. Look at any commercial theater or mixing studio and they use identical, vertically aligned L/C/R's for a reason (dispersal patterns, for one). I'd call up Ascend and ask them to swap out your current center for a single, matching Sierra tower. Since this is what you're using it for, I doubt they'd raise a stink. They want your continued business, I would assume.
As for your speakers and their locations... I would look closely
at the white papers put out by Dolby Labs on their website describing their object-oriented Atmos system (it's similar to DTS's MDA format). Yes, you guessed it... there is a new and more enveloping (and exciting) way of mixing and encoding soundtracks and it's just around the corner (for consumer use). Might as well consider the near future before everything is locked in stone and you have to rip stuff apart. Each speaker is classified as a "channel"... these formats are not confined to traditional channel amounts and are completely scalable from 2 to 9 to 11 to 22 to... (you get my point).http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/Dolby-Atmos-Cinema-Technical-Guidelines.pdf
For one, it might behoove you to have at least one pair of side surrounds for each row... or have the wiring in place to add some more HTM-200 SE's in those locations. As for the off-screen wide surrounds, you would more than likely place them between the front row side surrounds and the behind-the-screen L/R's. Heck, if you have room for two more frontal side speaker locations to make things more uniform in your side wall speaker placement... I'd place speaker cable there as well. Wouldn't hurt.
I would also drop another set of speaker cables behind the screen because there are optional left center and right center screen speakers with these formats. Those would have to be yet another pair of Sierra towers to maintain complete timbre matching. Too bad they don't have timbre matching Sierra surrounds or small, sealed Sierra monitors with bracket mounts to be used as surrounds so all your speakers could have matched.
The wall mounted surrounds (sides and rear) should also be aimed in accordance to Atmos recommendations.
Then consider putting speaker cable in the ceiling for left/right overhead speakers. You probably would want to consider at least two pairs of HTM-200's on angled brackets and safety cabling (don't want them dropping on your guests!), so you can position them in accordance with object-oriented speaker aiming requirements.
At least the wires would be in place... it's cheap peace of mind. The white papers lay out the recommended placements... just don't have to put in 64 speakers... if you don't want to!!
All speakers must be wired separately because they are all addressed separately (again, consider each speaker a channel) by the object rendering decoder. Because surround effects can be full frequency under Atmos and MDA specs., there may be surround subwoofer outputs. Be sure to place subwoofer cabling in the left/right rear of the room just in case.
Even though it's not out yet, you could still wire up most of the speakers for DTS Neo:X post processing for the time being. You're extra efforts won't go to waste.
Just thought you should know.Edited by Dan Hitchman - 3/3/13 at 8:43pm