I've run sealed subs and I've also run servo OB subs. Trust me, the servo OB subs are far, far, far better than sealed subs when you run OB mains.
Now, whether or not you want subs at all with the M3's is a separate question. The M3's might be all you need. If so, then great, no need for the extra expense of subs. But if you DO decide you want more bass, skip the box subs and go straight to the servo OB subs.