the new M- subs sw5500 and sw1600 are excellent and much improved from the previous M- subs; I would recommend them (presently have a sw5500) with 5200s in front; excellent voice match and more than enough punch;

I haven't heard the 3100 except a quick demo at CEDIA (poor acoustics), but they handled it very well - I liked them a lot. I use 5Ks as surrounds for 5200s; they are a good match and are full range for the music surround.

Although, I don't find many mixes of either music or movies that actually send full range to the surrounds; going forward, I think this will be done more and more, but realistically I think if you had a second sub in the rear and use a 3100, it would be equivalent - perhaps better; I'm not convinced that for the small amount of surround-bass that gets mixed is particularly directional, even when present;

just some rambling thoughts-

ken