The internal build quality of the NAD and Cambridge appears to be better. The Cambridge appears more well-thought out in that department. You are correct in that NAD is more transparent about their specs. In consideration of this last point, if someone held a gun to my head and I had to choose between all of these on potential sound quality alone, I would choose the NAD.
Along the lines of Ian's guidance, consider the following. In your 8x8 room, the M3s will give you an honest 97dBC of sound pressure level before you can perceive any distortion from the woofer. That's only 16 peak Watts. Perhaps, if you are not a discerning listener, you may not perceive any distortion until 103dBC. That's 64 peak Watts.
97dBC is nothing to be ashamed of. That is plenty. Given your posts on the M3 vs. the M5 and your concern to save some cash, I think either the Sony or Yammie will fill your needs while being easier on your wallet.
