My room is only 14 x 19 ... so the M3 would be a perfect fit. We will see.
Oh I'm pretty sure you have enough room for MPros. These aren't ported speakers, nor are they as warm as your S5's. I would love to have that size room with my MPros.
These beasts are fascinating in the way that they can change tonal balance in my system. I tinker a lot - with speaker positioning, power cords, room treatments, etc and am constantly amazed at how the character of the overall sound can change so much. There is no doubt that they have a leaner tonal balance than the S5 (I've had both overlap in my system for 8 months) but they make the S5 seem so veiled. These speakers truly are a window into the chain as much as the music.
I've had them where I was convinced they were just too forward sounding (almost bright) and I've had them where I was completely bored with them (no sparkle, air, etc.). Moving the speakers an inch or two, adjusting toe-in, changing a power cable or interconnect can make all this difference. In that sense, it might not be a plug-and-play type speaker, but I don't think any speaker this transparent will be. If all you do is listen to pop music you might be better off with the S5, but if you are into complex music (e.g. symphonic) these clever arrays of transducers will let equal parts drama and beauty escape effortlessly into your room with swirls of jaw-dropping goodness. The intricate detail of string sections in the upper frequencies is matched by the thunderous authority in the deep bass and everything in between.
All that said, it takes care and proper feeding to get right. I heard the MPros at Axpona 2015 and was horrified at how bad they sounded. If my pair was still on order at the time I would have been trying to cancel them. There were other speakers at that show that sounded so much better - no contest.