The minimum a listener needs for 2-channel is the M5. For 2.1, the M2 may actually be a better bet because the M5 has a bass bump that may make integration into some rooms a bit tough.
The M2 is really an M5 without a woofer so with a sub, I'd expect it to sound close to an M5. I say close because the M5 has a lower crossover to the tweeter and that may make the M5 highs slightly more dispersive.
I certainly won't miss the 150 although I know that slimmer profile was the only thing that worked for some applications.
As for the M50, I'm willing to bet an M2 with a sub will fit the same needs.
