Nuke -
I've attached a drawing of what I understand you're after. On one hand you're trying to fit as many of the M80 drivers as possible behind the screen, but on the other hand unless you place the screen near the floor, you'll need to elevate it with a stand or sub.
There are several goals to go for (many mentioned here) that you're balancing:
1) Screen height. There's no hard and fast rule, some strive for the bottom edge to 1/6 the image to be at eye level, but this depends on several things. Do you like to sit in the third row and look up? Are your seats reclining? Do you have rows of heads to need to overcome? Answers to this will get you the "higher" or "lower" vote regarding image. I have tall tower speakers, and generally lean towards a lower image - my eyeline is about 1/4 up.
2) Sound image height. Having three M80s grants you the incredible advantage of having a consistent sound height (not to mention tonality, wave size, polar and transient response, but I guess I mentioned it) across your fronts. Elevate the center too much and you'll be trading off one advantage of having an identical center channel. It'll still be way better than the typical off-screen nonidentical center channel, and way way way better than dual top/bottom MTM centers - typically a sloppy mess and worse than going without a center at all.
3) Not blocking drivers with the screen border. Try to get the non-AT screen border as out of the drivers way as possible. You'll be ok if you get it down in-between the woofers. Early reflections down that low are less audible. I have a customer's install on the press page where the side L/R had to have the frame in between the woofer and midrange.
Here's what I think you have:
Nuke's M80 trio of the Gods. Based on the feel of things, I would encourage you to think lower on the screen. You may not be able to hook it to the ceiling without chain or turnbuckles. You can cover with something (I've seen cut-off black socks work well, but I'm not going to officially recommend that or else my redneck score will go up).
Also, you may be able to elevate the L/R up a few inches. Building a nice heavy platform(s) for them can also improve the sound by decoupling them from the floor. A few speakers actually rely on floor-coupling, but it's a cheap audiophile trick that often tightens up the bass.
Then after those two, you may find your center channel stand height requirement shorter. Or sitting it on a sub would be pretty badass.
Let me know if you want the drawing tweaked to show changes. I actually do recommend heavy tow-in for the L/R if they're outside the screen. Try it - it's free and I find it typically widens the sweet spot and improves the imaging for the off-center seats.
Definitely keep the speakers at least many inches from any nearby wall. Just space them out as much as reasonable. Being closer than 3 feet to reflective surfaces will blur the transient response. The closer you place them to the wall, the more aggressive you should treat the reflection points with 4" (no less) foam. foambymail is a good source.
Cheers,
Chris
