Speaker Placement and Room Treatment - Feedback wanted.

The open baffle mids and highs will benefit from diffusion on the front-wall behind the speakers. But as millercarbon states, you're going to have issues with room modes in the bass (particularly around 35Hz and 70Hz). Whether these modes will cause a boost or cut in SPL depends on both the speaker and listener position. It's sometimes possible to fix this with eq, but only for a single listening position. You're much better off by adding bass absorption, and as millercarbon suggests, adding another sub or two in different positions which will reduce the amplitude of the bass peaks and dips at any given location.  

I'd consider adding a diffusor right behind each speaker, bass traps in the front and rear corners (or just the front if both is impractical). After that, adding absorption at the first reflection points on the side walls as well as the ceiling (particularly since you've got a fairly low ceiling) will help with imaging and dialog intelligibility.  You can get absorbers with scatter plates if you are worried about the room becoming too dead (which will depend on how absorptive your carpet and furniture is).

Well the length is almost exactly twice the height. All room have mode areas where frequencies reinforce or cancel. The high ones are no problem or are easily controlled. The lower once though you pretty much have to live with. There's tables you can calculate and map them out, see Robert Harley's The Complete Guide to High End Audio for a full length explanation of how to do it and what can be done to absorb. This is the most comprehensive yet easy to understand info around.

So you could do all that. Or you could just play some music or test tones and walk around and hear them for yourself. Either way what you are gonna find, a prominent bass hump almost impossible to remove. Because bass is long wave length, absorbing it takes an equally big trap, or whatever you want to call it. Harley has some really ingenious ideas like converting a closet into a bass trap. You could post diagrams and pics all day and still in the end you know your situation best so guess who gets to figure out what to do? You do.

Harley has what you need to know to be able to do that. Only thing he's missing, or maybe he's added it in the current edition, is info on distributed bass arrays.

The DBA is the closest thing there is to an answer to your (and everyone else's) problem. Multiple subs. The problem with modes is they form and are located relative to not only the room but the source of the wave. The usual one or two sub solution results in one or two great big modes which you have to try and smooth by moving them around, or with EQ, neither very good. With lots of subs though you get lots of modes. With more subs each one plays less loud, so the modes are smaller. With lots of subs they can be smaller, and where they go matters much less. Everyone who has tried this finds its the best they have heard.

As for your stereo speakers, the diagram looks pretty good. The normal process is to move them around looking for the best trade off between smooth bass response and imaging. Too close to walls bass is great but imaging suffers. And vice versa. Yours look about right but only you can tell and only by trying different locations.

Then when you're happy with that the next step is to fine tune. Tweak them to precisely equidistant and symmetrical toe-in. Center channel and surrounds are extra crap the home theater industry wants you to buy. When the stereo pair are correct you don't need no center speaker. And when the imaging is good the sound stage is so much more believable than any surround gimmick its not even funny. But most HT guys love doing what they're supposed to more than what actually works, so your call. Only mention since you said 2CH listening. And HT gear is absolute crap, SQ wise. Yes all of it.

When everything seems real close the last thing is get a couple sheets of Owens Corning acoustic panel, 2'x3'x1" cut it up and experiment to use it to damp first reflections. Use sparingly, or risk an over-damped room. The raw panels are light enough to tack on the wall with stick pins for testing. Then when you get it right wrap with some nice fabric and impress your friends with your professional looking home made acoustic panels.