I used to own a pair of M-3's, and the short answer is yes they will work quite well in your room and with your amp.
Now there is something you ought to know, because it doesn't show up in the specs. The M-3 is about 3 dB less efficient than the big full-size M-1's (which in turn is slightly less efficient than Maggie 3.6's, if that gives you a basis for comparison). The clarity and articulation is the same as the bigger models, the bass doesn't go quite as deep, and the presentation isn't quite as lush. On the other hand, it's easier to get good soundstaging out of the narrower panels (they radiate into about a 60 degree arc, and so have less sidewall interaction). Now the one thing that bugged me about the M-3's is that you lose the highs when you stand up. I'd suggest putting something under the front of the speaker to tip it back just a wee bit so you don't lose the highs.
Your Pass Labs amps should work quite well with them - that's a nice combination. In the same price ball park as the Pass, Brian and I both sell the 400-watt Parasound JC-1, which obviously we think is a winning combination.
Now I'm not trying to talk you out of your price range, but just to give you more information. If by chance you could stretch and get a pair of the M-2's, in my opinion that's probably Sound Lab's best "bang for the buck" (if you can say that about a speaker that retails for roughly the price of a Honda Civic). In terms of how rich and lush the overall presentation is, there's a bigger gap between the M-3 and the M-2 than between the M-2 and the M-1. Also, you get a bit better bass and a bit better efficiency. That being said, you can indeed live happily ever after with a pair of M-3's!
If you go with the M-3's, I'd say try to avoid using absorptive room treatments. You don't have as much reverberant energy, so use diffusion to best preserve what you do have (to a large extent, it's that tonally correct reverberant energy that gives Sound Labs their rich texture).
RWD, you are correct that Sound Labs don't interact with the ceiling (or floor) much. Basically, they beam like a searchlight in the vertical plane - which is why you lose the highs if your ears get above the top of the diaphragm. I've never set up a pair of Sound Labs in a room like yours, where they have to be placed within a 9 1/2 foot wide area, so after I post this I'll go scoot one of my A-1's over so that the two are within an area 9 1/2 feet wide and try listening from 8 -10 feet back. I'll post my findings later on. You've got me curious now!
I'd be more than happy to take a shot at any additional questions either of you might have.
Best wishes to you in your quest!